gdb: make interp::m_name an `const char *`

I realized that the memory for interp names does not need to be
allocated.  The name used to register interp factory functions is always
a literal string, so has static storage duration.  If we change
interp_lookup to pass that name instead of the string that it receives
as a parameter (which does not always have static storage duration),
then interps can simply store pointers to the name.

So, change interp_lookup to pass `factory.name` rather than `name`.
Change interp::m_name to be a `const char *` rather than an std::string.

Change-Id: I0474d1f7b3512e7d172ccd73018aea927def3188
Reviewed-By: Tom Tromey <tom@tromey.com>