[mips64] Fix ipc syscalls

Mips defines CONFIG_ARCH_WANT_IPC_PARSE_VERSION for both 32 and 64-bit.
See arch/mips/Kconfig. Other supported arches do it for 32-bit only.
This translates in having to pass IPC_64 flag for mips32 and mips64.
We use __mips__ to pass the flag, which is also defined for both 32 and
64-bit.

Change fixes bionic smoke tests (sys_msg, sys_sem, sys_shm) for mips64.

Test: bionic-unit-tests --gtest_filter="sys_*.smoke"
Change-Id: I918e4ffafd2002bb3e62ee252406746778100031