ld testsuite CFLAGS_FOR_TARGET

run_host_cmd adds $gcc_B_opt and $ld_L_opt to the command line if it
detects the program being run is a compiler.  Since the program being
run in lto.exp linking pr28138 is "sh", we need to add these by hand.
This isn't exactly as run_host_cmd does, as it lacks reordering of
any user -B option in $CC_FOR_TARGET, but it's better than ignoring
gcc_B_opt.  This fixes a mips64 testsuite fail.

ld_compile adds CFLAGS_FOR_TARGET and other flags as well, so there
is no need for the ld_compile command line to include
CFLAGS_FOR_TARGET.  Fixing this is just a tidy.

	* testsuite/ld-plugin/lto.exp: Add gcc_B_opt, CFLAGS_FOR_TARGET
	and $ld_L_opt to pr28138 link line.
	* testsuite/lib/ld-lib.exp (run_ld_link_tests): Don't pass
	unnecessary flags to ld_compile.
	(run_ld_link_exec_tests, run_cc_link_tests): Likewise.