runtime: pass correct size to malloc

In both cases we lie to malloc about the actual size that we need.
In panic we ask for less memory than we are going to use.
In slice we ask for more memory than we are going to use
(potentially asking for a fractional number of elements).
This breaks the new GC.

LGTM=khr
R=golang-codereviews, dave, khr
CC=golang-codereviews, rsc
https://golang.org/cl/116940043

src/pkg/runtime/panic.c

@@ -41,7 +41,7 @@ newdefer(int32 siz)
 	}
 	if(d == nil) {
 		// deferpool is empty or just a big defer
-		total = TOTALSIZE(siz);
+		total = runtime·roundupsize(TOTALSIZE(siz));
 		d = runtime·malloc(total);
 	}
 	d->siz = siz;

src/pkg/runtime/slice.goc

@@ -117,14 +117,18 @@ growslice1(SliceType *t, Slice x, intgo newcap, Slice *ret)
 
 	if(newcap1 > MaxMem/typ->size)
 		runtime·panicstring("growslice: cap out of range");
+	// Try to use all memory that malloc will give us...
 	capmem = runtime·roundupsize(newcap1*typ->size);
+	// ...but don't ask for fractional number of elements (that can confuse GC).
+	newcap1 = capmem/typ->size;
+	capmem = newcap1*typ->size;
 	flag = 0;
 	// Can't use FlagNoZero w/o FlagNoScan, because otherwise GC can scan unitialized memory.
 	if(typ->kind&KindNoPointers)
 		flag = FlagNoScan|FlagNoZero;
 	ret->array = runtime·mallocgc(capmem, (uintptr)typ|TypeInfo_Array, flag);
 	ret->len = x.len;
-	ret->cap = capmem/typ->size;
+	ret->cap = newcap1;
 	lenmem = x.len*typ->size;
 	runtime·memmove(ret->array, x.array, lenmem);
 	if(typ->kind&KindNoPointers)