Don't consider '-foffload-abi' in 'DEF_GOACC_BUILTIN', 'DEF_GOMP_BUILTIN'

Since Tom's PR64707 commit r220037 (Git commit
1506ae0e) "Make fopenmp an LTO option" as well
as PR64672 commit r220038 (Git commit a0c88d06)
"Make fopenacc an LTO option", we're now actually passing
'-fopenacc'/'-fopenmp' to the 'mkoffload's, which will pass these on to the
offload compilers.

	gcc/
	* builtins.def (DEF_GOACC_BUILTIN, DEF_GOMP_BUILTIN): Don't
	consider '-foffload-abi'.
	* common.opt (-foffload-abi): Remove 'Var', 'Init'.
	* opts.c (common_handle_option) <-foffload-abi> [ACCEL_COMPILER]:
	Ignore.