runtime: do not zero terminate strings

On top of "tiny allocator" (cl/38750047), reduces number of allocs by 1% on json.
No code must rely on zero termination. So will also make debugging simpler,
by uncovering issues earlier.

json-1
allocated                 7949686      7915766      -0.43%
allocs                      93778        92790      -1.05%
time                    100957795     97250949      -3.67%
rest of the metrics are too noisy.

LGTM=r
R=golang-codereviews, r, bradfitz, iant
CC=golang-codereviews
https://golang.org/cl/40370061

src/pkg/runtime/string.goc

@@ -46,10 +46,8 @@ gostringsize(intgo l)
 
 	if(l == 0)
 		return runtime·emptystring;
-	// leave room for NUL for C runtime (e.g., callers of getenv)
-	s.str = runtime·mallocgc(l+1, 0, FlagNoScan|FlagNoZero);
+	s.str = runtime·mallocgc(l, 0, FlagNoScan|FlagNoZero);
 	s.len = l;
-	s.str[l] = 0;
 	for(;;) {
 		ms = runtime·maxstring;
 		if((uintptr)l <= ms || runtime·casp((void**)&runtime·maxstring, (void*)ms, (void*)l))