ld: pass -g for ld-elf tests

The "DWARF parse during linker error" and "Build warn libbar.so" tests
require debug information.

configure defaults to "-O2 -g" but if overriding *FLAGS when building
tests, this might be lost. Explicitly pass -g given these tests require
it.

Originally reported downstream in Gentoo at https://bugs.gentoo.org/934149.

ld/
	* testsuite/ld-elf/dwarf.exp: Pass -g for "DWARF parse during linker error".
	* testsuite/ld-elf/shared.exp: Ditto for "Build warn libbar.so".