runtime: optimize permission changes with mprotect

On Linux, both mprotect() and mmap() acquire the mmap_lock (in writer mode),
posing scalability challenges.

The mmap_lock (formerly called mmap_sem) is a reader/writer lock that controls
access to a process's address space; before making changes there (mapping in a
new range, for example), the kernel must acquire that lock.

Page-fault handling must also acquire mmap_lock (in reader mode) to ensure that
the address space doesn't change in surprising ways while a fault is being resolved.

A process can have a large address space and many threads running (and incurring
page faults) concurrently, turning mmap_lock into a significant bottleneck.

While both mmap() and mprotect() are protected by the mmap_lock, the shorter
duration of mprotect system call, due to their simpler nature, results in a reduced
locking time for the mmap_lock.

Change-Id: I7f929544904e31eab34d0d8a9e368abe4de64637
GitHub-Last-Rev: 6f27a216b4
GitHub-Pull-Request: golang/go#65038
Reviewed-on: https://go-review.googlesource.com/c/go/+/554935
Reviewed-by: Michael Knyszek <mknyszek@google.com>
Reviewed-by: Mauri de Souza Meneguzzo <mauri870@gmail.com>
LUCI-TryBot-Result: Go LUCI <golang-scoped@luci-project-accounts.iam.gserviceaccount.com>
Reviewed-by: Michael Pratt <mpratt@google.com>

src/internal/runtime/syscall/defs_linux_mips64x.go

@@ -7,6 +7,7 @@
 package syscall
 
 const (
+	SYS_MPROTECT      = 5010
 	SYS_FCNTL         = 5070
 	SYS_EPOLL_CTL     = 5208
 	SYS_EPOLL_PWAIT   = 5272