Match library name prefixed with sysroot

Match library name prefixed with sysroot

We enable systemtap probe in glibc recently, and see the following gdb fail,

(gdb) set solib-absolute-prefix /.
...
Stopped due to shared library event:^M
  Inferior loaded /./foo/bar/gdb.base/break-probes-solib.so
...
(gdb) FAIL: gdb.base/break-probes.exp: run til our library loads (the program exited)

$binfile_lib is /foo/bar/gdb.base/break-probes-solib.so, but the
sysroot is prefixed in solib.c:solib_find, as comments described:

   Global variable GDB_SYSROOT is used as a prefix directory
   to search for shared libraries if they have an absolute path.

so the output becomes "/./foo/bar/gdb.base/break-probes-solib.so", which
is still correct.  However, the test repeatedly continue the program
and tries to match $binfile_lib, finally, the program exits and the
test fails.

This patch is to adjust the pattern to match $sysroot$binfile_lib
instead of $binfile_lib.

gdb/testsuite:

2014-11-28  Yao Qi  <yao@codesourcery.com>

	* gdb.base/break-probes.exp: Match library name prefixed with
	sysroot.