testsuite: prune DOS drive letter in test outputs

On DOS systems, absolute paths start with the drive letter. This can
trigger failures in the regexp from dump tests, especially for those
checking for warnings or errors. They are usually skipping everything
before the first ":" as it has to be the file path.
  | [^:]*: warning: ...

In order to avoid modifying many regexps to allow such drive letters,
prune them from all the outputs if they are found at the beginning of
a line.

binutils/ChangeLog:

	* testsuite/lib/binutils-common.exp (prune_dump_output): New
	(run_dump_test): Use it.

ld/ChangeLog:

	* testsuite/ld-elf/noinit-sections-2.l: Remove DOS drive letter
	handler.