ARM: only use one set of dummy FPA registers

ARM: only use one set of dummy FPA registers

All ARM cores need to provide obsolete FPA registers in their
GDB register dumps.  (Even though cores with floating point
support now generally use some version of VFP...)

Clean up that support a bit by sharing the same dummy registers,
and removing the duplicate copies.  Eventually we shouldn't need
to export those dummies.

(This makes the ARMv7-M support include the armv4_5 header, and
cleans up related #includes, but doesn't yet use anything from
there except those dummies.)

Signed-off-by: David Brownell <dbrownell@users.sourceforge.net>