Fix libbacktrace build on hppa-hpux.

	2020-02-29  John David Anglin  <danglin@gcc.gnu.org>

	Backport from mainline
	2019-09-26  Ian Lance Taylor  <iant@golang.org>

	PR libbacktrace/91908
	* pecoff.c (backtrace_initialize): Explicitly cast unchecked
	__sync_bool_compare_and_swap to void.
	* xcoff.c (backtrace_initialize): Likewise.