Mark functions/variables directly accessed by assembly as HIDDEN

Assembly code accesses symbols (variables/functions) such as x264_8_cabac_trnsition_unary and x264_10_cabac_encode_ue_bypass PC relatively, without going through GOT entries. This implies these symbols must be non-preemptible (cannot be provided by main executable or another DSO).

Any DSO (including libx264.so or application DSO linking libx264.a) linked from such object files requires -Bsymbolic to make the symbols non-preemptible. However, -Bsymbolic is dangerous as it breaks C++ semantics about address uniqueness of inline functions, type_info (used by exceptions). This means libx264.a can not be safely linked into a C++ application DSO.

This patch adds the macro HIDDEN and marks symbols referenced by assembly as HIDDEN. Without -Bsymbolic, the following command won't fail with link errors like "relocation R_X86_64_PC32 cannot be used against symbol ...":

ld --whole-archive libx264.a -shared -o /dev/null

Another advantage is that even in PIC mode, a C compiler will access such symbols directly instead of going through a GOT entry.