Extend recognized types of SDT probe's arguments

This commit is actually an update to make the parser in
gdb/stap-probe.c be aware of all the possible prefixes that a probe
argument can have.  According to the section "Argument Format" in:

  <https://sourceware.org/systemtap/wiki/UserSpaceProbeImplementation>

The bitness of the arguments can be 8, 16, 32 or 64 bits, signed or
unsigned.  Currently GDB recognizes only 32 and 64-bit arguments.
This commit extends this.  It also provides a testcase, only for
x86_64 systems.

gdb/
2014-05-02  Sergio Durigan Junior  <sergiodj@redhat.com>

	* stap-probe.c (enum stap_arg_bitness): New enums to represent 8
	and 16-bit signed and unsigned arguments.  Update comment.
	(stap_parse_probe_arguments): Extend code to handle such
	arguments.  Use warning instead of complaint to notify about
	unrecognized bitness.

gdb/testsuite/
2014-05-02  Sergio Durigan Junior  <sergiodj@redhat.com>

	* gdb.arch/amd64-stap-optional-prefix.S (main): Add several
	probes to test for bitness recognition.
	* gdb.arch/amd64-stap-optional-prefix.exp
	(test_probe_value_without_reg): New procedure.
	Add code to test for different kinds of bitness.