ARM: pass 'struct reg *' to register r/w routines

ARM: pass 'struct reg *' to register r/w routines

Implementations need to access the register struct they modify;
make it easier and less error-prone to identify the instance.
(This removes over 10% of the ARMV4_5_CORE_REG_MODE nastiness...)

Plus some minor fixes noted when making these updates:  ARM7/ARM9
accessor methods should be static; don't leave CPSR wrongly marked
"dirty"; note significant XScale omissions in register handling;
and have armv4_5_build_reg_cache() record its result.

Rename "struct armv4_5_core_reg" as "struct arm_reg"; it's used
for more than those older architecture generations.

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