[builtins] Add COMPILER_RT_BUILTINS_HIDE_SYMBOLS
On Android, when the builtins are linked into a binary, they are typically linked using -Wl,--exclude-libs so that the symbols aren't reexported. For the NDK, compiler-rt's default behavior (build the builtins archive with -fvisibility=hidden) is better so that builtins are hidden even without -Wl,--exclude-libs.
Android needs the builtins with non-hidden symbols only for a special case: for backwards compatibility with old binaries, the libc.so and libm.so DSOs in the platform need to export some builtins for arm32 and 32-bit x86. See D56977.
Control the behavior with a new flag, `COMPILER_RT_BUILTINS_HIDE_SYMBOLS`, that behaves similarly to the `*_HERMETIC_STATIC_LIBRARY` in libunwind/libcxx/libcxxabi, so that Android can build a special builtins variant for libc.so/libm.so.
Unlike the hermetic flags for other projects, this new flag is enabled by default.