__pthread_internal_remap_stack_with_mte: stop setting stack top guard to RW

[rdevshp]

pthread_create.cpp __allocate_thread_mapping does the following:

// Layout from the end of the mmap-ed region (before the top PTHREAD_GUARD_SIZE):
//
// [ PTHREAD_GUARD_SIZE ]
// [ libgen_buffers_padded_size (for dedicated page(s) for libgen buffers) ]
// [ static_tls_layout_size ]
// [ thread_page_size (for pthread_internal_t) ]
// [ gap_size (for (random) guard page(s)) ]
// [ stack_size ]
// [ stack_guard_size ]

ThreadMapping result = {};
result.mmap_base = space;
result.mmap_size = mmap_size;
result.mmap_base_unguarded = space + stack_guard_size;
result.mmap_size_unguarded = mmap_size - stack_guard_size - PTHREAD_GUARD_SIZE;

This means that after

mprotect(t->mmap_base_unguarded, t->mmap_size_unguarded,
                 PROT_READ | PROT_WRITE | PROT_MTE)

is executed in __pthread_internal_remap_stack_with_mte, the stack top guard is set to RW.
This PR proposes a fix for this issue.

[rdevshp]

Superseded by #75 .