Cherry-picked the following commits:
e56f9639 define GET_GOT_DEFINED if not previously defined
a4b47b89 x86inc.asm: only set visibility for chromium builds
a97c83f7 Only use .text sections for aout
9679be4b Use .text instead of .rodata on macho
7e065cd5 Copy PIC handling code from x86_abi_support
71ccd103 Set 'private_extern' visibility for macho targets
1be46ef6 Expand PIC default to macho64 and respect CONFIG_PIC from libvpx
4de9641f Use libvpx defines to set name mangling rules
2d6393a9 Customize x86inc.asm for libvpx
2497356d Update x86inc.asm from x264

Change-Id: I2fe309bf7b5cea866177cec954024ced59aed789