Detect availability of PR_SET_VMA at runtime (fixes build with musl libc)

The build would fail on musl-based systems with the following error:

    gcc -c -fno-strict-overflow -Wsign-compare -DNDEBUG -g -O3 -Wall    -std=c11 -Wextra -Wno-unused-parameter -Wno-missing-field-initializers -Wstrict-prototypes -Werror=implicit-function-declaration -fvisibility=hidden  -I./Include/internal -I./Include/internal/mimalloc  -I. -I./Include    -DPy_BUILD_CORE -o Objects/obmalloc.o Objects/obmalloc.c
    In file included from ./Include/internal/pycore_mmap.h:16,
                     from Objects/obmalloc.c:5:
    /usr/include/sys/prctl.h:88:8: error: redefinition of 'struct prctl_mm_map'
       88 | struct prctl_mm_map {
          |        ^~~~~~~~~~~~
    In file included from ./Include/internal/pycore_mmap.h:15:
    /usr/include/linux/prctl.h:134:8: note: originally defined here
      134 | struct prctl_mm_map {
          |        ^~~~~~~~~~~~
    make: *** [Makefile:3388: Objects/obmalloc.o] Error 1

This is due to Python including both <linux/prctl.h> and <sys/prctl.h> (which on
glibc imports the definitions from <linux/prctl.h> and on musl includes all the
definitions itself).
Given that Python is not the Linux kernel, <sys/prctl.h> should be included.

Another issue of the previous implementation and the configure check was that
the `_PyAnnotateMemoryMap()` function would only be enabled if the linux-headers
on the build host are recent enough to contain the `PR_SET_VMA_ANON_NAME` macro.

However the resulting binaries might be run on older kernels lacking the
functionality or be built on older systems but run on newer kernels.

With this patch, the function is always enabled and the magic numbers are
included with Python, so that the `prctl()` is always executed and
feature detection is essentially done at runtime.

When run on a kernel too old to support `PR_SET_VMA_ANON_NAME`,
`mmap.mmap.set_name()` will silently do nothing.
When the kernel is new enough but `CONFIG_ANON_VMA_NAME` is not enabled,
`mmap.mmap.set_name()` will raise an `OSError`, e.g.:

    >>> mm.set_name("hello")
    Traceback (most recent call last):
      File "<python-input-5>", line 1, in <module>
        mm.set_name("hello")
        ~~~~~~~~~~~^^^^^^^^^
    OSError: [Errno 22] Invalid argument

Author: sideeffect42

Include/internal/pycore_mmap.h

@@ -11,12 +11,17 @@ extern "C" {
 
 #include "pycore_pystate.h"
 
-#if defined(HAVE_PR_SET_VMA_ANON_NAME) && defined(__linux__)
-#  include <linux/prctl.h>
-#  include <sys/prctl.h>
+#if defined(__linux__)
+
+#include <sys/prctl.h>
+
+#ifndef PR_SET_VMA
+#  define PR_SET_VMA 0x53564d41
+#endif
+#ifndef PR_SET_VMA_ANON_NAME
+#  define PR_SET_VMA_ANON_NAME 0
 #endif
 
-#if defined(HAVE_PR_SET_VMA_ANON_NAME) && defined(__linux__)
 static inline int
 _PyAnnotateMemoryMap(void *addr, size_t size, const char *name)
 {
@@ -33,12 +38,15 @@ _PyAnnotateMemoryMap(void *addr, size_t size, const char *name)
     }
     return 0;
 }
+
 #else
+
 static inline int
 _PyAnnotateMemoryMap(void *Py_UNUSED(addr), size_t Py_UNUSED(size), const char *Py_UNUSED(name))
 {
     return 0;
 }
+
 #endif
 
 #ifdef __cplusplus

configure

@@ -5728,15 +5728,6 @@ AC_CHECK_DECLS([UT_NAMESIZE],
                          [Define if you have the 'HAVE_UT_NAMESIZE' constant.])],
               [],
               [@%:@include <utmp.h>])
-# musl libc redefines struct prctl_mm_map and conflicts with linux/prctl.h
-AS_IF([test "$ac_cv_libc" != musl], [
-AC_CHECK_DECLS([PR_SET_VMA_ANON_NAME],
-              [AC_DEFINE([HAVE_PR_SET_VMA_ANON_NAME], [1],
-                         [Define if you have the 'PR_SET_VMA_ANON_NAME' constant.])],
-              [],
-              [@%:@include <linux/prctl.h>
-               @%:@include <sys/prctl.h>])
-])
 # check for openpty, login_tty, and forkpty
 
 AC_CHECK_FUNCS([openpty], [],

configure.ac

@@ -224,10 +224,6 @@
 /* Define to 1 if you have the <db.h> header file. */
 #undef HAVE_DB_H
 
-/* Define to 1 if you have the declaration of 'PR_SET_VMA_ANON_NAME', and to 0
-   if you don't. */
-#undef HAVE_DECL_PR_SET_VMA_ANON_NAME
-
 /* Define to 1 if you have the declaration of 'RTLD_DEEPBIND', and to 0 if you
    don't. */
 #undef HAVE_DECL_RTLD_DEEPBIND
@@ -1003,9 +999,6 @@
 /* Define if your compiler supports function prototype */
 #undef HAVE_PROTOTYPES
 
-/* Define if you have the 'PR_SET_VMA_ANON_NAME' constant. */
-#undef HAVE_PR_SET_VMA_ANON_NAME
-
 /* Define to 1 if you have the 'pthread_condattr_setclock' function. */
 #undef HAVE_PTHREAD_CONDATTR_SETCLOCK