Fix KERNEL_HEADERS_ARCH for mips and x86.

arm and arm64 have distinct headers, but mips and mip64, and x86 and x86_64
use headers that work for both widths. So where arm/arm64 need to handle the
second architecture case specially, all we need to do for the others is
hard-code the name. (x86_64.mk already hard-codes x86; we need to change
x86.mk for the case where we're building the 32-bit binaries for a mixed
system. mips64.mk doesn't exist yet, but when it does, it'll hard-code
just plain "mips" too.)

Change-Id: Ia6b9f77b4eb2c78729b454045875c409e0ea8197