Fix probe-related internal error on AIX

-- Initial message by Tom Tromey:

While testing on AIX, I happened to notice an internal error coming
from parse_probes.  This happens because there are no probes defined
on this platform.  This patch fixes the problem by changing an assert
into an ordinary error, and then changing the relevant caller to cope.

This fixes a few tests on AIX; also regtested on x86-64 Fedora 18.

-- Followup by Sergio Durigan Junior:

By reading the patch (and the original code), I found it a little bit
obscure, so I took the liberty to try to improve it.  Here's the patch.
Could you please take a look and see if it works on AIX (and also if you
like the approach)?

gdb/
2014-03-20  Tom Tromey  <tromey@redhat.com>
	    Sergio Durigan Junior  <sergiodj@redhat.com>

	* probe.c (parse_probes): Turn assert into an ordinary error.
	* break-catch-throw.c (re_set_exception_catchpoint): Ignore
	exceptions when parsing probes.  Rearrange the code for clarity.
gas/
	* config/tc-mips.h (DIFF_EXPR_OK, CFI_DIFF_EXPR_OK): Define.
	* config/tc-mips.c (md_pcrel_from): Remove error message.
	(md_apply_fix): Convert PC-relative BFD_RELOC_32s to
	BFD_RELOC_32_PCREL.  Report a specific error message for unhandled
	PC-relative expressions.  Handle BFD_RELOC_8.

gas/testsuite/
	* gas/all/gas.exp: Remove XFAIL of forward.d for MIPS.
	* gas/mips/pcrel-1.s, gas/mips/pcrel-1.d, gas/mips/pcrel-2.s,
	gas/mips/pcrel-2.d, gas/mips/pcrel-3.s, gas/mips/pcrel-3.l,
	gas/mips/pcrel-4.s, gas/mips/pcrel-4-32.d, gas/mips/pcrel-4-n32.d,
	gas/mips/pcrel-4-64.d: New tests.
	* gas/mips/mips.exp: Run them.
	* gas/mips/lui-2.l: Tweak error message for line 7.

ld/testsuite/
	* ld-elf/merge.d: Remove MIPS XFAIL.
Avoid using the ISO C99 `z' formatted output modifier

	* mi/mi-interp.c (mi_memory_changed): Avoid using the ISO C99
	`z' formatted output modifier.
Add opcode relaxation for rl78-elf

This patch adds initial in-gas opcode relaxation for the rl78
backend.  Specifically, it checks for conditional branches that
are too far and replaces them with inverted branches around longer
fixed branches.
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