Add '_' in the match pattern.

I was looking at GDB testcase results for arm-eabi target with qemu and
noticed that register groups returned by the qemu can have '_' in the
name e.g. 'cp_regs'. The reggroups.exp fails to recognize that as group
name. Fixed by adding '_' in the pattern.

2018-09-20  Hafiz Abid Qadeer  <abidh@codesourcery.com>

	gdb.base/reggroups.exp (fetch_reggroups): Add '_' in match pattern.
2 files changed