gdb: make gdbarch_printable_names return a vector

gdb: make gdbarch_printable_names return a vector

I noticed that gdbarch_selftest::operator() leaked the value returned by
gdbarch_printable_names.  Make gdbarch_printable_names return an
std::vector and update callers.  That makes it easier for everyone
involved, less manual memory management.

Change-Id: Ia8fc028bdb91f787410cca34f10bf3c5a6da1498