Arm: Fix out of range conditional branch (PR/24991)

Arm: Fix out of range conditional branch (PR/24991)

The fix for PR12848 introduced an off by one error in the mask, this corrected
the negative overflows but not the positive overflows.  As a result the
conditional branch instructions accepted a too wide positive immediate which
resulted in it corrupting the instruction during encoding.

The relocation I believe has been incorrectly named, to be consistent with the
other relocations it should have been named BRANCH21 which is why the masks for
it are confusing.

I've replaced the masks with a function out_of_range_p which should make it
harder to make such mistakes.

The mask for BL/BLX on Armv6t+ is also wrong, the extended range is 25-bits
and so the mask should be checking for 24-bits for positive overflow.

gas/ChangeLog:

	PR gas/24991
	* config/tc-arm.c (out_of_range_p): New.
	(md_apply_fix): Use it in BFD_RELOC_THUMB_PCREL_BRANCH9,
	BFD_RELOC_THUMB_PCREL_BRANCH12, BFD_RELOC_THUMB_PCREL_BRANCH20,
	BFD_RELOC_THUMB_PCREL_BRANCH23, BFD_RELOC_THUMB_PCREL_BRANCH25
	* testsuite/gas/arm/pr24991.d: New test.
	* testsuite/gas/arm/pr24991.l: New test.
	* testsuite/gas/arm/pr24991.s: New test.