Arm: Fix thumb2 PLT branch offsets.

When I previously changed these offsets I had incorrectly used an offset of -2
for this Thumb2 PLT.  Unfortunately because we had no tests for this PLT I had
missed that the result was incorrect.

This patch fixes the offset to PC .-4 so that it correctly addresses the
previous instruction and adds a test for this PLT stub.

bfd/ChangeLog:

	* elf32-arm.c (elf32_thumb2_plt_entry): Fix PC-rel offset.

ld/ChangeLog:

	* testsuite/ld-arm/arm-elf.exp (thumb-plt): New.
	* testsuite/ld-arm/thumb-plt.d: New test.
	* testsuite/ld-arm/thumb-plt.s: New test.