gdb: remove VALUE_REGNUM, add value::regnum

Remove VALUE_REGNUM, replace it with a method on struct value.  Set
`m_location.reg.regnum` directly from value::allocate_register_lazy,
which is fine because allocate_register_lazy is a static creation
function for struct value.

Change-Id: Id632502357da971617d9dce1e2eab9b56dbcf52d
7 files changed