[dev.garbage] runtime: use sys.Ctz64 intrinsic

Our compilers now provides instrinsics including
sys.Ctz64 that support CTZ (count trailing zero)
instructions. This CL replaces the Go versions
of CTZ with the compiler intrinsic.

Count trailing zeros CTZ finds the least
significant 1 in a word and returns the number
of less significant 0s in the word.

Allocation uses the bitmap created by the garbage
collector to locate an unmarked object. The logic
takes a word of the bitmap, complements, and then
caches it. It then uses CTZ to locate an available
unmarked object. It then shifts marked bits out of
the bitmap word preparing it for the next search.
Once all the unmarked objects are used in the
cached work the bitmap gets another word and
repeats the process.

Change-Id: Id2fc42d1d4b9893efaa2e1bd01896985b7e42f82
Reviewed-on: https://go-review.googlesource.com/21366
Reviewed-by: Austin Clements <austin@google.com>

src/runtime/mheap.go

@@ -145,7 +145,7 @@ type mspan struct {
 	// Cache of the allocBits at freeindex. allocCache is shifted
 	// such that the lowest bit corresponds to the bit freeindex.
 	// allocCache holds the complement of allocBits, thus allowing
-	// ctz64 (count trailing zero) to use it directly.
+	// ctz (count trailing zero) to use it directly.
 	// allocCache may contain bits beyond s.nelems; the caller must ignore
 	// these.
 	allocCache uint64