aix: remove libgomp and libatomic archives before creating FAT archives

AIX caches shared objects in archives with read-other permission.
libgomp and libatomic might be in use during the build or testing, which
may cause archiver operations on them to fail.  This patch adjusts the
Makefile fragments to delete the library archives before creating fresh
archives containing both the 32 bit and 64 bit shared objects.

libatomic/ChangeLog:

2020-10-11  Clement Chigot  <clement.chigot@atos.net>

	* config/t-aix: Delete and recreate libatomic before creating
	FAT library.

libgomp/ChangeLog:

2020-10-11  Clement Chigot  <clement.chigot@atos.net>

	* config/t-aix: Delete and recreate libgomp before creating
	FAT library.