[llvm-libgcc] Don't export __{,de}register_frame_info* and __register_frame_table

These symbols are used in the absence of PT_GNU_EH_FRAME, for ld
--no-eh-frame-hdr (gcc -static default).

libunwind defines these empty functions when
`defined(_LIBUNWIND_BUILD_ZERO_COST_APIS) && defined(_LIBUNWIND_SUPPORT_DWARF_UNWIND) && defined(_LIBUNWIND_SUPPORT_FRAME_APIS)`.
They should be perceived as Mac OS X workarounds.

For Linux, GCC crtbeginT.o (for -static) and clang_rt.crtbegin.o contain
weak references to `__{,de}register_frame_info`. Dynamically linked
executables will either not reference `__{,de}register_frame_info` (using
libgcc) or reference them as weak symbols (using compiler-rt).

Therefore, not defining these symbols is backward compatible.

Reviewed By: phosek

Differential Revision: https://reviews.llvm.org/D158241