New make target 'check-perf' and new dir gdb.perf

We add a new dir gdb.perf in testsuite for all performance tests.
However, current 'make check' logic will either run dejagnu in
directory testsuite or iterate all gdb.* directories which has *.exp
files.  Both of them will run tests in gdb.perf.  We want to achieve:

 1) typical 'make check' should not run performance tests.  In each perf
    test case, GDB_PERFTEST_MODE is checked.  If it doesn't exist, return.
 2) run perf tests easily.  We add a new makefile target 'check-perf'.

gdb:

2013-11-06  Yao Qi  <yao@codesourcery.com>

	* Makefile.in (check-perf): New target.

gdb/testsuite:

2013-11-06  Yao Qi  <yao@codesourcery.com>

	* Makefile.in (check-perf): New target.
	* configure.ac (AC_OUTPUT): Output Makefile in gdb.perf.
	* configure: Re-generated.
	* gdb.perf/Makefile.in: New.