Get triple from the make variable CC_TARGET_TRIPLE (#1558)

Allow overriding the target triple used by the Xcode Swift toolchain
through the CC_TARGET_TRIPLE environment variable. As
cc_toolchain.target_gnu_system_name is getting deprecated, and as rules
based toolchain currently don't offer the option to set it anymore, this
offers a way by which someone can define a rules based toolchain and
still have working swift rules.

Author: ma-oli

swift/toolchains/swift_toolchain.bzl

@@ -416,7 +416,7 @@ def _swift_toolchain_impl(ctx):
         os = ctx.attr.os,
         target_system_name = cc_toolchain.target_gnu_system_name,
     )
-    target_triple = target_triples.normalize_for_swift(
+    target_triple = ctx.var.get("CC_TARGET_TRIPLE") or target_triples.normalize_for_swift(
         target_triples.parse(target_system_name),
     )
 

swift/toolchains/xcode_swift_toolchain.bzl

@@ -627,7 +627,7 @@ def _xcode_swift_toolchain_impl(ctx):
     apple_toolchain = apple_common.apple_toolchain()
     cc_toolchain = find_cpp_toolchain(ctx)
 
-    target_triple = target_triples.normalize_for_swift(
+    target_triple = ctx.var.get("CC_TARGET_TRIPLE") or target_triples.normalize_for_swift(
         target_triples.parse(cc_toolchain.target_gnu_system_name),
     )