Move `compute_probe_arg' and `compile_probe_arg' to probe.c

This patch moves the `compute_probe_arg' and `compile_probe_arg' functions
from stap-probe.c to probe.c.  The rationale is that it is reasonable to
assume that all backends will provide the `$_probe_argN' convenience
variables, and that the user must be placed on the PC of the probe when
requesting that information.  The value and type of the argument can still be
determined by the probe backend via the `pops->evaluate_probe_argument' and
`pops->compile_to_ax' handlers.

Note that a test in gdb.base/stap-probe.exp had to be adjusted because the "No
SystemTap probe at PC" messages are now "No probe at PC".

gdb/ChangeLog:

2015-02-17  Jose E. Marchesi  <jose.marchesi@oracle.com>

	* probe.c (compute_probe_arg): Moved from stap-probe.c
	(compile_probe_arg): Likewise.
	(probe_funcs): Likewise.
	* stap-probe.c (compute_probe_arg): Moved to probe.c.
	(compile_probe_arg): Likewise.
	(probe_funcs): Likewise.

gdb/testsuite/ChangeLog:

2015-02-17  Jose E. Marchesi  <jose.marchesi@oracle.com>

	* gdb.base/stap-probe.exp (stap_test): Remove "SystemTap" from
	expected message when trying to access $_probe_* convenience
	variables while not on a probe.