bpf: fix calculation when deciding to relax branch

bpf: fix calculation when deciding to relax branch

In certain cases we were calculating the jump displacement incorrectly
when deciding whether to relax a branch.  This meant for some branches,
such as a very long backwards conditional branch, relaxation was not
done when it should have been.  The result was to error later, because
the actual jump displacement was too large to fit in the original
instruction.

This patch fixes up the displacement calculation so that those branches
are correctly relaxed and no longer result in an error.  In addition, it
changes md_convert_frag to install fixups for the JAL instructions in
the resulting relaxations rather than encoding the displacement value
directly.

gas/
	* config/tc-bpf.c (relaxed_branch_length): Correct displacement
	calculation when relaxing.
	(md_convert_frag): Likewise.  Install fixups for JAL
	instructions resulting from relaxation.
	* testsuite/gas/bpf/jump-relax-ja-be.d: Correct and expand test.
	* testsuite/gas/bpf/jump-relax-ja.d: Likewise.
	* testsuite/gas/bpf/jump-relax-ja.s: Likewise.
	* testsuite/gas/bpf/jump-relax-jump-be.d: Likewise.
	* testsuite/gas/bpf/jump-relax-jump.d: Likewise.
	* testsuite/gas/bpf/jump-relax-jump.s: Likewise.