testsuite: Update scan-lto-assembler

scan-lto-assembler was looking for the old dump names instead of
the new ones.  There are only two uses of this:

    gcc.target/nios2/custom-fp-lto.c
    gcc.target/aarch64/pr70044.c

Both of them use single source files, so I think they both want
the same behaviour.

gcc/testsuite/
	* lib/scanasm.exp (scan-lto-assembler): Update expected name
	of dump file.