x86_abi_support: do not decorate coff functions
x86_abi_support: do not decorate coff functions

:private_extern only applies to macho. Match x86inc.asm logic:
%if FORMAT_ELF
  global %2:function hidden
%elif FORMAT_MACHO
  global %2:private_extern
%else
  global %2
%endif

May fix a build issue on windows:
vp8/encoder/x86/block_error_sse2.asm:18: error:
  COFF format does not support any special symbol types

BUG=webm:1679

Change-Id: I7e1f4043b064a04752d1cedd030cbe7f5461fe40

webmproject/libvpx,ShiftMediaProject/libvpx,webmproject/libvpx,ShiftMediaProject/libvpx,webmproject/libvpx,webmproject/libvpx,webmproject/libvpx,ShiftMediaProject/libvpx,ShiftMediaProject/libvpx,webmproject/libvpx,ShiftMediaProject/libvpx