COFF build-id writes uninitialised data to file

COFF build-id writes uninitialised data to file

1) The first write in write_build_id wrote rubbish past the struct
external_IMAGE_DEBUG_DIRECTORY, which was later overwritten with
correct data.  No user visible problem there, except that tools like
valgrind complain.
2) The size for the pdb name was incorrectly calculated.

	* emultempl/pe.em (write_build_id): Write the debug directory,
	not the entire section contents.
	(setup_build_id): Add size for the base name of pdb_name, not
	the full path.
	* emultempl/pep.em: Likewise.
	* testsuite/ld-pe/pdb2-section-contrib.d: Update.