Introduce ax_raw_byte and use it

Introduce ax_raw_byte and use it

This patch was taken directly from Pedro's branch.

ax_simple is used to append an agent expression operator to an agent
expression string.  Therefore, it takes an enum agent_op as input.
There is an instance where it's called to append a raw byte, unrelated
to the enum.  It makes the build fail in C++ mode.

This patch introduces ax_raw_byte for that purpose and uses it.

gdb/ChangeLog:

	* ax.h (ax_raw_byte): New declaration.
	* ax-general.c (ax_raw_byte): New function.
	(ax_simple): Use ax_raw_byte.
	* ax-gdb.c (gen_printf): Likewise.