cmd/link: set S_ATTR_PURE_INSTRUCTIONS bit for text section on darwin
cmd/link: set S_ATTR_PURE_INSTRUCTIONS bit for text section on darwin

Mac otool and llvm-objdump distinguishes a Mach-O section is
text or data by looking at S_ATTR_PURE_INSTRUCTIONS bit. Without
this bit it thinks our function symbols are data, not functions.
Set this bit for text section to make otool/objdump happy.

Fixes #24706.

Change-Id: I5236482cb9a72474c23fbea0f35d5b5cc8491ea4
Reviewed-on: https://go-review.googlesource.com/105256
Run-TryBot: Cherry Zhang <d62e63aa42ce272d7b6a5055d97e942b33a34679@google.com>
TryBot-Result: Gobot Gobot <66cb808b70d30c07676d5e946fee83fd561249e5@golang.org>
Reviewed-by: Ian Lance Taylor <87e9c6d529889242b7e184afb632328636553ab4@golang.org>

nawawi/go,golang/go,cloudflare/go,akutz/go,CAFxX/go,nawawi/go,cloudflare/go,nawawi/go,CAFxX/go,CAFxX/go,golang/go,CAFxX/go,golang/go,akutz/go,CAFxX/go,josharian/go,cloudflare/go,nawawi/go,akutz/go,josharian/go,akutz/go,nawawi/go,golang/go,CAFxX/go,josharian/go,josharian/go,cloudflare/go,josharian/go,cloudflare/go,golang/go,josharian/go,josharian/go,akutz/go,nawawi/go,golang/go,josharian/go,cloudflare/go,cloudflare/go,akutz/go,CAFxX/go,akutz/go,golang/go,nawawi/go,nawawi/go,CAFxX/go,golang/go,akutz/go,cloudflare/go