test/codegen: port math/bits.RotateLeft tests to codegen

Only RotateLeft{64,32} were tested, and just for ppc64. This CL adds
tests for RotateLeft{64,32,16,8} on arm64 and amd64/386, for the cases
where the calls are actually instrinsified.

RotateLeft tests (the last ones for math/bits functions) are deleted
from asm_test.

This CL also adds a space between the "//" and the arch name in the
comments, to uniform this file to the style used in all the other
files.

Change-Id: Ifc2a27261d70bcc294b4ec64490d8367f62d2b89
Reviewed-on: https://go-review.googlesource.com/99596


Reviewed-by: Giovanni Bajo <rasky@develer.com>