[msan] Use SIGHUP instead of SIGUSR1 in test.

Apparently, SIGUSR1 does not work on x86_64+ArchLinux for some reason.

For more details, see:
https://groups.google.com/forum/#!topic/llvm-dev/4Ag1FF4M2Dw


git-svn-id: https://llvm.org/svn/llvm-project/compiler-rt/trunk@214289 91177308-0d34-0410-b5e6-96231b3b80d8