gdbserver: Simplify handling of ZMM registers.

- Reuse num_xmm_registers directly for the count of ZMM0-15 registers
  as is already done for the YMM registers for AVX rather than using
  a new variable that is always the same.

- Replace 3 identical variables for the count of upper ZMM16-31
  registers with a single variable.  Make use of this to merge
  various loops working on the ZMM XSAVE region so that all of the
  handling for the various sub-registers in this region are always
  handled in a single loop.

- While here, fix some bugs in i387_cache_to_xsave where if
  X86_XSTATE_ZMM was set on i386 (e.g. a 32-bit process on a 64-bit
  kernel), the -1 register nums would wrap around and store the value
  of GPRs in the XSAVE area.  This should be harmless, but is
  definitely odd.  Instead, check num_zmm_high_registers directly when
  checking X86_XSTATE_ZMM and skip the ZMM region handling entirely if
  the register count is 0.

Approved-By: Simon Marchi <simon.marchi@efficios.com>