[GAS/ARM] Fix expansion of ldr pseudo instruction

[GAS/ARM] Fix expansion of ldr pseudo instruction

The LDR rX, =cst pseudo-instruction suffers from two issues for loading
integer constants in Thumb mode:

- movs is used if the constant and register can be encoded using that
  instruction which leads to unexpected behavior due to its flag-setting
  behavior
- mov.w, movw and mvn are used for r13 (sp) and r15 (pc) but these
  encoding are marked as UNPREDICTABLE

This patch fixes those issues and update testing accordingly.

2017-04-24  Thomas Preud'homme  <thomas.preudhomme@arm.com>

gas/
	* config/tc-arm.c (move_or_literal_pool): Remove code generating MOVS.
	Forbid MOV.W and MOVW if destination is SP or PC.
	* testsuite/gas/arm/thumb2_ldr_immediate_highregs_armv6t2.s: Explain
	expectation of LDR not generating a MOVS for low registers and small
	constants.  Add tests of MOVW generation.
	* testsuite/gas/arm/thumb2_ldr_immediate_highregs_armv6t2.d: Update
	expected disassembly.