gfortran: Get asan library from TEST_ALWAYS_FLAGS

gfortran: Get asan library from TEST_ALWAYS_FLAGS

Update gfortran_target_compile to get the newly built asan library from
TEST_ALWAYS_FLAGS to avoid:

/usr/bin/ld: cannot find libasan_preinit.o: No such file or directory
/usr/bin/ld: cannot find -lasan
collect2: error: ld returned 1 exit status
compiler exited with status 1
FAIL: gfortran.dg/asan/pointer_assign_16.f90 -fsanitize=address  -O0  (test for excess errors)

	PR fortran/94788
	* lib/gfortran.exp (gfortran_target_compile): Get asan library
	from TEST_ALWAYS_FLAGS.