mm: implement the membarrier(2) syscall

[gburd]

What

Implements the membarrier(2) syscall, which was missing entirely (not in the
syscall table, no libc entry, no prototype). Runtimes that use it for cheap
asymmetric synchronization (Go, .NET, some JVMs, RCU-style user libraries) could
not use it on OSv.

How

OSv is a single process whose threads all share one address space, so a "global"
and a "private" membarrier are equivalent here. The expedited barrier forces
every other CPU through an IPI: mmu::flush_tlb_all() already broadcasts an IPI
and waits for each CPU to run its handler (a full memory barrier on that CPU),
and we add a local seq_cst fence. This is a correct superset of what
membarrier promises (it also flushes TLBs, which is harmless), and reuses a
proven, arch-portable primitive rather than adding a dedicated per-arch IPI (x64
inter_processor_interrupt vs aarch64 sgi_interrupt). A lighter dedicated IPI
can replace it if membarrier ever shows up hot in a profile.

• MEMBARRIER_CMD_QUERY returns the supported command mask.
• REGISTER_{GLOBAL,PRIVATE}_EXPEDITED are accepted as no-ops (any thread may
  issue the barrier).
• {GLOBAL, GLOBAL_EXPEDITED, PRIVATE_EXPEDITED} run the barrier.
• SYNC_CORE / RSEQ / CPU-flag variants and unknown commands return EINVAL,
  as does a nonzero flags argument.

Wired as syscall SYS_membarrier (adding __NR_membarrier / SYS_membarrier
for x86-64) with a tracepoint, plus the sys/membarrier.h header with the
command enum and prototype, and the symbol exported from libc.so.6 and
ld-musl.so.1.

Testing

tests/tst-membarrier.cc covers QUERY, register, the three barrier commands,
and the EINVAL paths. Passes on OSv under KVM with 1, 2 and 4 vCPUs (exercising
the cross-CPU IPI broadcast).

[gburd]

Superseded by #1434 (rebased onto current master; GitHub auto-closed this one when the branch was updated). Please review #1434 instead.