runtime: don't use twice the memory with grsec-like kernels

grsec needs the FIXED flag to be provided to mmap, which
works now.  That said, when the allocation fails to be made
in the specific address, we're still given back a writable
page.  This change will unmap that page to avoid using
twice the amount of memory needed.

It'd also be pretty easy to avoid the extra system calls
once we detected that the flag is needed, but I'm not sure
if that edge case is worth the effort.

R=golang-dev, rsc
CC=golang-dev
https://golang.org/cl/4634086

src/pkg/runtime/linux/mem.c

@@ -91,6 +91,9 @@ runtime·SysMap(void *v, uintptr n)
 		if(p != v && addrspace_free(v, n)) {
 			// On some systems, mmap ignores v without
 			// MAP_FIXED, so retry if the address space is free.
+			if(p > (void*)4096) {
+				runtime·munmap(p, n);
+			}
 			p = runtime·mmap(v, n, PROT_READ|PROT_WRITE|PROT_EXEC, MAP_ANON|MAP_FIXED|MAP_PRIVATE, -1, 0);
 		}
 		if(p == (void*)ENOMEM)