target/arm: Stop assuming DBGDIDR always exists

The AArch32 DBGDIDR defines properties like the number of
breakpoints, watchpoints and context-matching comparators.  On an
AArch64 CPU, the register may not even exist if AArch32 is not
supported at EL1.

Currently we hard-code use of DBGDIDR to identify the number of
breakpoints etc; this works for all our TCG CPUs, but will break if
we ever add an AArch64-only CPU.  We also have an assert() that the
AArch32 and AArch64 registers match, which currently works only by
luck for KVM because we don't populate either of these ID registers
from the KVM vCPU and so they are both zero.

Clean this up so we have functions for finding the number
of breakpoints, watchpoints and context comparators which look
in the appropriate ID register.

This allows us to drop the "check that AArch64 and AArch32 agree
on the number of breakpoints etc" asserts:
 * we no longer look at the AArch32 versions unless that's the
   right place to be looking
 * it's valid to have a CPU (eg AArch64-only) where they don't match
 * we shouldn't have been asserting the validity of ID registers
   in a codepath used with KVM anyway

Signed-off-by: Peter Maydell <peter.maydell@linaro.org>
Reviewed-by: Richard Henderson <richard.henderson@linaro.org>
Message-id: 20200214175116.9164-11-peter.maydell@linaro.org