Skip gdb ifunc tests on targets that don't support this feature.

Skip gdb ifunc tests on targets that don't support this feature.

2018-10-03  Sandra Loosemore  <sandra@codesourcery.com>

	* lib/gdb.exp (skip_ifunc_tests): New.
	* gdb.base/gnu-ifunc.exp: Skip if no ifunc support.  Handle
	other compile failures.
	* gdb.compile/compile-ifunc.exp: Skip if no ifunc support.