Remove srcdir from x86 testcase "source:" lines

It's wrong to have ${srcdir} in run_dump_test "source:" lines, as
run_dump_test adds $srcdir/$subdir/ to the line passed to the shell
except when the source path starts with "./".  The tests work
currently because the shell expands ${srcdir} to an empty string.

	PR 31728
	* testsuite/ld-i386/code16.d: Correct "source:".
	* testsuite/ld-x86-64/code16.d: Likewise.
	* testsuite/ld-x86-64/rela.d: Likewise.