[MIPS,MIPS64] Add asm/reg.h and mcontext_t struct for MIPS64

Cherry-pick changes:
    https://android-review.googlesource.com/#/c/146049/
    https://android-review.googlesource.com/#/c/147258/

Change-Id: I14555ed033a0d80f2b290e7424ba50c031209df3
Reviewed-on: https://chromium-review.googlesource.com/266842
Reviewed-by: Ross McIlroy <rmcilroy@chromium.org>
Tested-by: Ross McIlroy <rmcilroy@chromium.org>