Enable conditional breakpoints for targets that support software single step in GDBServer

Enable conditional breakpoints for targets that support software single step in GDBServer

This patch enables support for conditional breakpoints if the target supports
software single step.

This was disabled before as the implementations of software single step were too
simple as discussed in
https://sourceware.org/ml/gdb-patches/2015-04/msg01110.html.

Since these issues are now fixed support can be added back.

New tests passing :
PASS: gdb.base/cond-eval-mode.exp: set breakpoint condition-evaluation
target and related...

No regressions, tested on ubuntu 14.04 ARMv7 and x86.
With gdbserver-{native,extended} / { -marm -mthumb }

gdb/gdbserver/ChangeLog:

	* server.c (handle_query): Call target_supports_software_single_step.