Add arm_sync_icache() and arm_drain_writebuf() sysarch syscall wrappers.

NetBSD and OpenBSD have libc wrapper functions for the ARM_SYNC_ICACHE and
ARM_DRAIN_WRITEBUF sysarch operations. This change adds compatible functions
to our library. This should make it easier for various upstream sources to
support *BSD operating systems with a single variation of cache maintence
code in tools like interpreters and JIT compilers.

I consider the argument types passed to arm_sync_icache() to be especially
unfortunate, but this is intended to match the other BSDs.

Differential Revision: https://reviews.freebsd.org/D20906