Optional matching for run_dump_test

Optional matching for run_dump_test

This adds an alternative to using #... in dump files, useful where we
only want to allow specific extra output.  DW_CFA_nop in CIEs and FDEs
to pad out to required alignment (larger for 64-bit than 32-bit) is
an example where these optional match patterns are useful.

binutils/
	* testsuite/lib/binutils-common.exp (regexp_diff): Support #?REGEXP.
ld/
	* testsuite/ld-elf/eh4.d: Match optional padding DW_CFA_nop in FDEs.
	* testsuite/ld-elf/eh5.d: Likewise, and extra CIEs emitted on
	embedded targets.