ld: Update mixed LTO and non-LTO relocatable output tests

ld: Update mixed LTO and non-LTO relocatable output tests

Since mixed LTO and non-LTO relocatable output is only supported on ELF
platforms, limit these tests to ELF targets.  Since powerpc64 elfv1
defines a function symbol on its procedure descriptor, which is in a
data section, rather than on the code for that function, allow both D
and T for nm test on mixed object.

	* testsuite/ld-plugin/lto.exp: Limits  mixed LTO and non-LTO
	relocatable output tests to ELF targets.  Allow both D and T for
	nm test on mixed object.

Signed-off-by: H.J. Lu <hjl.tools@gmail.com>