Implement --cpumask to mask out CPU features

Fixes #198.

Usage: --cpumask=0xf or --cpumask=ssse3 will select up to and
including SSSE3 (so SSE+SSE2+SSE3+SSSE3).
12 jobs for master in 5 minutes and 25 seconds (queued for 2 seconds)
Status Job ID Name Coverage
  Build
passed #241569
amd64 debian
build-debian

00:00:33

passed #241573
debian aarch64
build-debian-aarch64

00:01:50

passed #241574
debian aarch64
build-debian-aarch64-clang-5

00:01:28

passed #241570
amd64 debian
build-debian-static

00:00:35

passed #241576
debian aarch64
build-debian-werror

00:00:31

passed #241575
macos
build-macos

00:00:42

passed #241571
win32
build-win32

00:00:28

passed #241572
win64
build-win64

00:00:36

 
  Test
passed #241577
amd64 debian
test-debian

00:00:52

passed #241578
amd64 debian
test-debian-asan

00:02:10

passed #241579
amd64 debian
test-debian-msan

00:01:04

passed #241580
amd64 debian
test-debian-ubsan

00:01:26