runtime: implement xadduintptr and update system mstats using it

The motivation is that sysAlloc/Free() currently aren't safe to be
called without a valid G, because arm's xadd64() uses locks that require
a valid G.

The solution here was proposed by Dmitry Vyukov: use xadduintptr()
instead of xadd64(), until arm can support xadd64 on all of its
architectures (not a trivial task for arm).

Change-Id: I250252079357ea2e4360e1235958b1c22051498f
Reviewed-on: https://go-review.googlesource.com/9002
Reviewed-by: Dmitry Vyukov <dvyukov@google.com>

src/runtime/export_test.go

@@ -21,6 +21,7 @@ var F64toint = f64toint
 var Entersyscall = entersyscall
 var Exitsyscall = exitsyscall
 var LockedOSThread = lockedOSThread
+var Xadduintptr = xadduintptr
 
 var FuncPC = funcPC
 
@@ -129,3 +130,5 @@ var Write = write
 
 func Envs() []string     { return envs }
 func SetEnvs(e []string) { envs = e }
+
+var BigEndian = _BigEndian