runtime: clean up power-of-two rounding code with align functions

This change renames the "round" function to the more appropriately named
"alignUp" which rounds an integer up to the next multiple of a power of
two.

This change also adds the alignDown function, which is almost like
alignUp but rounds down to the previous multiple of a power of two.

With these two functions, we also go and replace manual rounding code
with it where we can.

Change-Id: Ie1487366280484dcb2662972b01b4f7135f72fec
Reviewed-on: https://go-review.googlesource.com/c/go/+/190618
Reviewed-by: Austin Clements <austin@google.com>
Reviewed-by: Keith Randall <khr@golang.org>

src/runtime/stack.go

@@ -337,7 +337,7 @@ func stackalloc(n uint32) stack {
 	}
 
 	if debug.efence != 0 || stackFromSystem != 0 {
-		n = uint32(round(uintptr(n), physPageSize))
+		n = uint32(alignUp(uintptr(n), physPageSize))
 		v := sysAlloc(uintptr(n), &memstats.stacks_sys)
 		if v == nil {
 			throw("out of memory (stackalloc)")