Probe catch syscall support

In 82075af2 (Implement 'catch syscall'
for gdbserver), only x86 is supported, but the test can still be run
on other linux targets, like aarch64 and ppc, with native-gdbserver.
This causes many new fails.

This patch removes the check on isnative and on target triplets.
Instead, we can insert catch point, and resume the program to see whether
catch syscall is supported or not.

gdb/testsuite:

2016-06-28  Yao Qi  <yao.qi@linaro.org>

	* gdb.base/catch-syscall.exp: Remove check on isnative and target
	triplets.  Start gdb, execute catch syscall, and continue.  Check
	gdb's output to determine catch syscall is supported.