ld/testsuite: consistently add board_ldflags when linking with GCC

Currently, the functions checking if the compiler is available or if a
feature is available add both board_cflags and board_ldflags.
However, functions running the tests only retrieve board_cflags. This
can lead to unexpected errors when mandaratory flags are defined in
board_ldflags and not board_cflags.

ld/ChangeLog:

	* testsuite/ld-unique/unique.exp: Add board_ldflags when
	linking with GCC.
	* testsuite/lib/ld-lib.exp: Likewise.