ld script fill pattern expression

It turns out we do need to backtrack when parsing after all.  The
fill_opt component in the section rule swiches to EXPRESSION and back
to SCRIPT, and to find the end of an expression it is necessary to
look ahead one token.

	* ldgram.y (section): Throw away lookahead NAME token.
	(overlay_section): Likewise.
	* testsuite/ld-elf/overlay.t: Add fill pattern on overlays.
	Test fill pattern before stupidly named normal sections too,
	and before /DISCARD/.