[Safestack] Fix the canary test to catch the libc's message regarding stack smashing

By default glibc writes its diagnostics directly to tty so the `2>&1 |`
redirection in the test doesn't catch the *** stack smashing detected ***
message, which in turn breaks printing the lit's progress bar. By defining
the LIBC_FATAL_STDERR_ environment variable we force glibc to direct
diagnostic messages to stderr.

Differential Revision: https://reviews.llvm.org/D32599

llvm-svn: 302628