ld insert_os_after ignoring first assignment

ld insert_os_after ignoring first assignment

Some time ago the pr19593 test was xfailed for alpha.  This turned out
to be the wrong course of action since the test exposed a bug in
orphan section placement.  On alpha, orphan sections were being
inserted before ". = SIZEOF_HEADERS" due to the test having two
assignments, and on alpha, an output section statement from
-Ttext-segment being passed to ld.

	* ldlang.c (insert_os_after): Clear ignore_first on assignment to
	dot, not any assignment.  Clear ignore_first on output section
	statement contents too.
	* testsuite/ld-elf/pr19539.d: Remove alpha xfail.