Allowing pairing mips64r6 with mips32r1 and r2

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