[gdb/testsuite] Fix regexp for reg value in jit-reader.exp

[gdb/testsuite] Fix regexp for reg value in jit-reader.exp

On openSUSE Leap 15.1 (as well as on Fedora-x86_64-m64 buildbot) I see:
...
FAIL: gdb.base/jit-reader.exp: with jit-reader: after mangling: current frame: info registers
...

The problem is that r10 is printed signed:
...
r10            0xffffffffffffffb0  -80^M
...
but the regexp expects a signed value:
...
            "r10            $hex +$decimal" \
...

Fix this by allowing signed values.

Tested on x86_64-linux.

gdb/testsuite/ChangeLog:

2019-10-16  Tom de Vries  <tdevries@suse.de>

	* gdb.base/jit-reader.exp: Allow non-pointer registers to be printed
	as signed.

Change-Id: Ie494d24fad7a9af7ac6bfaf731c4aa04f1333830