ld: Add -fno-lto to linker tests

LTO can be used to build binutils with

$ CC="gcc -flto -ffat-lto-objects -Wl,--as-needed" CXX="g++ -flto -ffat-lto-objects -Wl,--as-needed" .../configure

But not all linker tests are compatible with LTO.  Pass -fno-lto to CC
to disable LTO on linker tests by default.  -flto is passed explicitly
to CC in linker LTO tests.

	* testsuite/ld-elf/indirect.exp: Append -fno-lto to CC.
	* testsuite/ld-elfvers/vers.exp: Likewise.
	* testsuite/ld-elfweak/elfweak.exp: Likewise.
	* testsuite/ld-ifunc/ifunc.exp: Likewise.
	* testsuite/ld-plugin/lto.exp (no_lto): New.
	Add $no_lto to build pr15146c.so.
	* testsuite/lib/ld-lib.exp (at_least_gcc_version): Filter out
	-Wl,xxx options.
	(check_gcc_plugin_enabled): Likewise.
	(run_ld_link_exec_tests): Prepend -fno-lto to $cflags.
	(run_cc_link_tests): Likewise.