gdb: move struct reggroup into reggroups.h header

gdb: move struct reggroup into reggroups.h header

Move 'struct reggroup' into the reggroups.h header.  Remove the
reggroup_name and reggroup_type accessor functions, and just use the
name/type member functions within 'struct reggroup', update all uses
of these removed functions.

There should be no user visible changes after this commit.