LD/testsuite: ld-lib.exp: Fix a typo, s/regexp/regex/

Match `$check_ld(source)' against "regex" rather than "regexp" in
reporting, correcting test framework diagnostics, e.g.:

tmpdir/undefined.o: In function `foo':
(.text+0x0): undefined reference to `bar'
failed with: <tmpdir/undefined.o: In function `foo':
(.text+0x0): undefined reference to `bar'>, no expected output
tmpdir/undefined.o: In function `foo':
(.text+0x0): undefined reference to `bar'
PASS: MIPS undefined reference

(current) vs:

tmpdir/undefined.o: In function `foo':
(.text+0x0): undefined reference to `bar'
failed with: <tmpdir/undefined.o: In function `foo':
(.text+0x0): undefined reference to `bar'>, expected: <\A[^\n]*\.o: In function `foo':\n\(\.text\+0x0\): undefined reference to `bar'\Z>
tmpdir/undefined.o: In function `foo':
(.text+0x0): undefined reference to `bar'
PASS: MIPS undefined reference

(corrected).  No functional change.

	ld/
	* ld/testsuite/lib/ld-lib.exp (run_dump_test): Fix a typo,
	s/regexp/regex/.