[compiler-rt] During profile flushing, setup SIGKILL mask earlier (#68622)

In multi threaded application, it is possible for one thread to
terminate the program while another is flushing profile information. We
setup a signal mask to delay SIGKILL so that we can safely flush the
profile.

This patch setups the mask earlier: it reduces the window during which a
SIGKILL can end the computation prematurely.

This fixes https://bugzilla.mozilla.org/show_bug.cgi?id=1850940

, where a
profiled firefox was encountering several:

LLVM Profile Error: Failed to write file "default_*.profraw": Broken
pipe

Co-authored-by: serge-sans-paille <sguelton@mozilla.com>