Restore original GDB prompt in define.exp

Restore original GDB prompt in define.exp

define.exp will fail on a GDB which has set a custom prompt to identify
itself.  This is because the test resets the prompt to a hard coded
"(gdb)" but then verifies the success of this against the value in
$gdb_prompt, which is set to the custom prompt.

The original approach to fix this involved resetting the prompt to
$gdb_prompt rather than a hard coded "(gdb)". However it was noted during
review that $gdb_prompt is a regular expression rather than a string.
This is problematic because in general the prompt would be reset to a
regular expression rather than an instance of a string accepted by said
regular expression.

The fix used in this commit avoids the above issue by capturing the
literal prompt from running "show prompt" and uses this literal to
restore the previous prompt.

Regression tested with GCC 7.3.0 on x86_64, ppc64le, aarch64.

gdb/testsuite/ChangeLog:
2019-07-10  Richard Bunt  <richard.bunt@arm.com>
	Stephen Roberts  <stephen.roberts@arm.com>

	* gdb.base/define.exp: Restore original prompt.