[MIPS] Resolve issues in building ASAN for N32 ABI

Building the compiler-rt's AddressSanitizer for
the n32 MIPS ABI currently fails, due to a few reasons:

    - defined(__mips64), which is set solely based on
    the architecture type (32-bit/64-bit), was still used
    in some places. Therefore, defined(__mips64) is swapped
    with SANITIZER_MIPS64, which takes the ABI into account
    as well - defined(__mips64) && _MIPS_SIM == ABI64.
    - The n32 ABI still uses 64-bit *Linux* system calls,
    even though the word size is 32 bits.
    - After the transition to canonical system calls (D124212),
    the n32 ABI still didn't use them, even though they
    are supported.

Differential Revision: https://reviews.llvm.org/D127098