[sanitizer] Disable ASLR for release_shadow_space

On aarch64 with kernel 4.12.13 the test sporadically fails with

RSS at start: 1564, after mmap: 103964, after mmap+set label: 308768, \
after fixed map: 206368, after another mmap+set label: 308768, after \
munmap: 206368
release_shadow_space.c.tmp: [...]/release_shadow_space.c:80: int \
main(int, char **): Assertion `after_fixed_mmap <= before + delta' failed.

It seems on some executions the memory is not fully released, even
after munmap.  And it also seems that ASLR is hurting it by adding
some fragmentation, by disabling it I could not reproduce the issue
in multiple runs.