checkasm: replace rand() with xorshift xor128

Reduces the aarch64 checkasm run time from 22s to 2.4s.
12 jobs for master in 4 minutes and 41 seconds (queued for 1 second)
Status Job ID Name Coverage
  Build
passed #241853
amd64 debian
build-debian

00:00:35

passed #241857
debian aarch64
build-debian-aarch64

00:01:26

passed #241858
debian aarch64
build-debian-aarch64-clang-5

00:01:06

passed #241854
amd64 debian
build-debian-static

00:00:31

passed #241860
debian aarch64
build-debian-werror

00:00:32

passed #241859
macos
build-macos

00:00:25

passed #241855
win32
build-win32

00:00:28

passed #241856
win64
build-win64

00:00:38

 
  Test
passed #241861
amd64 debian
test-debian

00:00:43

passed #241862
amd64 debian
test-debian-asan

00:02:12

passed #241863
amd64 debian
test-debian-msan

00:00:57

passed #241864
amd64 debian
test-debian-ubsan

00:01:18