Ensure ld testsuite gcc -B options precede $CC -B options

Ensure ld testsuite gcc -B options precede $CC -B options

Various ld-elf/shared.exp and ld-plugin/lto.exp tests simply appended
the testsuite -B options intended to force gcc use the linker under test.
This fails if $CC itself has -B options, as when setting CC to run gcc
out of a build directory.  Net result is that tests were run using
the gcc build dir collect-ld.

	* config/default.exp: Don't make tmpdir/gas.  Put as symlink into
	tmpdir/ld.
	(gcc_gas_flag, gcc_ld_flag): Delete.
	(gcc_B_opt, ld_L_opt): New globals.
	ld-elf/shared.exp: Remove all refs to gcc_gas_flag and gcc_ld_flag.
	ld-plugin/lto.exp: Likewise.
	lib/ld-lib.exp (run_host_cmd): Add gcc_B_opt and ld_L_opt here.
	(ld_simple_link): Remove -B handling now that this is done in
	run_host_cmd.  Simplify.
	(default_ld_compile): Simplify.
	(check_lto_available): Use run_host_cmd_yesno.
	(check_lto_shared_available): Likewise.