[gdb/testsuite] Fix regexp in py-rbreak.exp

[gdb/testsuite] Fix regexp in py-rbreak.exp

This fails for me on openSUSE leap 15.0:
...
FAIL: gdb.python/py-rbreak.exp: check number of returned breakpoints is 11
...

The rbreak "" command expects 11 breaks, but I see two extra for
__libc_csu_fini and __libc_csu_init:
...
Breakpoint 13 at 0x4005b0: file elf-init.c, line 106.^M
Breakpoint 14 at 0x400540: file elf-init.c, line 68.^M
...

This patch fixes the failing test by excluding functions starting with an
underscore.

Tested on x86_64-linux.

2018-08-04  Tom de Vries  <tdevries@suse.de>

	* gdb.python/py-rbreak.exp: Fix rbreak regexp.