Use std.math.isPowerOfTwo across std lib

[squeek502]

Noticed that std.math.isPowerOfTwo got added in #2525 and so I went through and tried to find all instances of that logic and replace it with a call to that function.

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Only instance I found that I didn't replace is this one:

zig/std/special/compiler_rt/udivmod.zig

Line 67 in b7811d3

if ((d[high] & (d[high] - 1)) == 0) {

because it doesn't currently @import("std") and I wasn't sure if that was okay to add that to that file.

EDIT: Nevermind, it actually did @import("std"), just not at the top. Made that file use std.math.isPowerOfTwo as well.

EDIT#2: Left compiler_rt/udivmod.zig unchanged, see comments.