Make stap-probe.c:stap_parse_register_operand's "regname" an std::string

Make stap-probe.c:stap_parse_register_operand's "regname" an std::string

[ Backport of master commit 677052f2a5. ]

This patch simplifies the code of
stap-probe.c:stap_parse_register_operand by making "regname" an
std::string.  No functionality change.

I'm this code's maintainer, so I'm pushing this as it's a fairly
trivial patch.

gdb/ChangeLog:
2019-05-16  Sergio Durigan Junior  <sergiodj@redhat.com>

	* stap-probe.c (stap_parse_register_operand): Make "regname" an
	"std::string", simplifying the algorithm.