SH5 compact register numbering in gcc -> gdb interface

[Joern Rennecke]

> compact reg current #   sh5 equiv    proposed #
> -----------+-----------+------------+------------------
> T            242        R19           19
> GBR          238        R16           16

P.S.:
SH[1-4] gcc and gdb currently disagree about the meaning of
numbers 18 and 19.
   gcc gdb
17 t   gbr
18 gbr vbr

This shouldn't really matter since these registers should
not appear in debug information with the ABIs currently
in use.  It is confusing, however, that gcc pretends that
this is part of the interface.  I therefore propose to remove
mappings for T and GDB from SH_DBX_REGISTER_NUMBER.
	
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