testsuite: Make check-function-bodies work with LTO

This patch tries to make check-function-bodies automatically
choose between reading the regular assembly file and reading the
LTO assembly file.  There should only ever be one right answer,
since check-function-bodies doesn't make sense on slim LTO output.

Maybe this will turn out to be impossible to get right, but I'd like
to try at least.

gcc/testsuite/
	* lib/scanasm.exp (check-function-bodies): Look in ltrans0.ltrans.s
	if the test appears to be using LTO.