Stop generating GNU build notes for linkonce sections.

gas	* write.c (maybe_generate_build_notes): Generate notes on a
	per-code-section basis.  Skip linkonce sections.

ld	* testsuite/ld-elf/notes.exp: New file: Run new test.
	* testsuite/ld-elf/note1_1.s: New file: Source file for test.
	* testsuite/ld-elf/note1_2.s: New file: Source file for test.
	* testsuite/ld-elf/note1.r: New file: Expected readelf output.