On mips64r6 builds, the shell variable ARCH_MIPS_REV6 is not
cleared before compiling 2nd arch parts. This bug is harmless when
2nd arch is mips32r6 but it omits needed mips-specific assembly
files when compiling libagl and libpixelflinger for mips32r1 or r2.
Clearing the variable is impractical. Using TARGET_2ND_ARCH qualifiers
would work, but Arm and x86 use a simpler method with distinct ARCH_ARM
and ARCH_ARM64 symbols. ARCH_MIPS_REV6 is used in two places.
Both controll 32-bit parts only, so neither place needs adjustment.
Change-Id: Id1ea5e8b6f8666d9df219fa9ef41834bd31f5dd8
47b57e6bd2