[ARM] Add linker support for ARMv8-R

[ARM] Add linker support for ARMv8-R

=== Context ===

This patch is part of a patch series to add support for ARMv8-R
architecture. Its purpose is to add support for ARMv8-R in the linker.

=== Patch description ===

This patch is composed of 3 changes:

1) The main change is the addition of the logic for merging a file whose
Tag_CPU_arch build attribute is 15 (ARMv8-R). Namely, all pre-ARMv8 are
merged into ARMv8-R as well as ARMv8-R itself. ARMv8-A (14) merges into
ARMv8-A. ARMv8-M Baseline (16) and Mainline (17) are not allowed to
merge merge with ARMv8-R. Note that merging only occurs if the two
profiles are identical or one is S (Application or Realtime) and the
other is R.

2) using_thumb_only, using_thumb2_bl, using_thumb2 and arch_has_arm_nop
are updated according to capabilities of ARMv8-R and their BFD_ASSERT
updated to reflect that the logic is valid for ARMv8-R.

3) 2 build attribute merging tests are added to test the first change.

2017-06-24  Thomas Preud'homme  <thomas.preudhomme@arm.com>

bfd/
	* elf32-arm.c (using_thumb_only): Update list of architectures in
	BFD_ASSERT for which the logic is valid.
	(using_thumb2_bl): Likewise.
	(using_thumb2): Likewise and return true for ARMv8-R.
	(arch_has_arm_nop): Likewise.
	(tag_cpu_arch_combine): New v8r table for ARMv8-R Tag_CPU_arch
	merging logic.  Update commentis for value 15 of v8m_baseline,
	v8m_mainline and v4t_plus_v6_m arrays.  Use v8r array to decide
	merging of value 15 of Tag_CPU_arch.

ld/
	* testsuite/ld-arm/arm-elf.exp (EABI attribute merging 11): New test.
	(EABI attribute merging 12): Likewise.
	* testsuite/ld-arm/attr-merge-11a.s: New file.
	* testsuite/ld-arm/attr-merge-11b.s: New file.
	* testsuite/ld-arm/attr-merge-11.attr: New file.
	* testsuite/ld-arm/attr-merge-12a.s: New file.
	* testsuite/ld-arm/attr-merge-12b.s: New file.
	* testsuite/ld-arm/attr-merge-12.attr: New file.