Fix ld --gc-section segfault with ARMv8-M entry function in absolute section

Fix ld --gc-section segfault with ARMv8-M entry function in absolute section

bfd/
2016-09-14  Thomas Preud'homme  <thomas.preudhomme@arm.com>

	* elf32-arm.c (elf32_arm_gc_mark_extra_sections): Only mark section
	not already marked.

ld/
2016-09-14  Thomas Preud'homme  <thomas.preudhomme@arm.com>

	* testsuite/ld-arm/cmse-veneers.s: Add a test for ARMv8-M Security
	Extensions entry functions in absolute section.
	* testsuite/ld-arm/cmse-veneers.rd: Adapt expected output accordingly.