How do I replace DEPRECATED_TM_FILE?

[Joern Rennecke]

I've read that instead of setting DEPRECATED_TM_FILE, we should add new
members to the target vector.  However, how is the target vector supposed to
be initialized differently for different subtargets?

E.g. arc700 can both be used in an 'embedded' configuration and as a linux
target.  The register numbers in gdb are different for these configurations.
The register numbers used to be defined in separate DEPRECATED_TM_FILE
files.
I can add xxx_regnum members to the target vector and ARC_XX_REGNUM
accessor macros, but somehow I have to decide in arc_gdbarch_init how I am
supposed to initialize the target vector.