libiberty: sync with gcc

Import the following commits from GCC as of r16-6728-g7c3584be8c8806:
	07df546fb90 libiberty/testsuite: make test-pexecute's -t option a little more useful
	0c775dcf3e7 libiberty: Make `objalloc_free' `free'-like WRT null pointer
3 files changed