Give externally-provided symbol declarations default visibility

[rvandermeulen]

Abseil forward-declares a few symbols that are provided by something outside the object being linked:

Symbol(s)	File	Provided by
14x Annotate* race annotations	absl/base/dynamic_annotations.h	sanitizer runtime
__tsan_read1	absl/synchronization/mutex.cc	sanitizer runtime
__mmap2	absl/base/internal/direct_mmap.h	Bionic

None of these has a public header to include — none of the Annotate* functions nor __tsan_read1 appears in any compiler-rt sanitizer/*.h, and __mmap2 is a private Bionic symbol — so declaring them locally is the only option. But a local declaration inherits whatever visibility is in effect at that point.

When the translation unit is compiled with -fvisibility=hidden and abseil is linked into a shared library, those references become hidden undefined symbols. They then cannot be resolved from the sanitizer runtime (which lives in the executable) or from libc, and the link fails:

ld.lld: error: undefined hidden symbol: AnnotateIgnoreReadsBegin
ld.lld: error: undefined hidden symbol: AnnotateIgnoreWritesBegin
ld.lld: error: undefined hidden symbol: AnnotateIgnoreWritesEnd
ld.lld: error: undefined hidden symbol: AnnotateIgnoreReadsEnd
ld.lld: error: undefined hidden symbol: __tsan_read1
ld.lld: error: undefined hidden symbol: __mmap2

This wraps each declaration in #pragma GCC visibility push(default) / pop.

Verification

Compiling all ~163 abseil sources with -fsanitize=thread and a tree-wide #pragma GCC visibility push(hidden), then scanning every object for hidden undefined symbols: before this change the four Annotate* and __tsan_read1 are GLOBAL HIDDEN UND; after, every __tsan_* and Annotate* reference is GLOBAL DEFAULT UND. The only hidden undefined symbols remaining are AbslInternal* ones that abseil defines itself, which is correct.

For __mmap2, on a 32-bit Android shared-library link:

without the pragma:  ld.lld: error: undefined hidden symbol: __mmap2
with the pragma:     4: FUNC GLOBAL DEFAULT UND __mmap2@LIBC

Notes

• Every affected site is already inside a Bionic or sanitizer guard, so the GCC/Clang-only pragma does not reduce portability.
• The __mmap2 path only compiles on 32-bit architectures, which is why this is rarely hit — Chromium's Android builds are 64-bit. We hit it on 32-bit ARM.
• Found while updating the copy of abseil vendored into Firefox, which builds with -fvisibility=hidden tree-wide.