gdb: rename offset to high bits in ymm registers

gdb: rename offset to high bits in ymm registers

The xsave_ymm_avx512_offset data structure contains the xsave
offset to the upper 128 bits of a ymm register.  Similarly, for zmm this
offset is described by xsave_avx512_zmm_h_offset, h indicating the
high bits.  This commit renames the xsave_ymm_avx512_offset to
xsave_ymm_h_avx512_offset - as well as the associated define from
XSAVE_YMM_AVX512_ADDR to XSAVE_YMM_H_AVX512_ADDR - to make this
more consistent.

Note, that the regnum defines already included the 'h' for ymm, like
I387_YMM16H_REGNUM and I387_YMMH_AVX512_END_REGNUM.

Co-authored-by:  Nils-Christian Kempke  <nils-christian.kempke@intel.com>
Approved-By: Andrew Burgess <aburgess@redhat.com>