Adds DWARF register number and support for AVX-512 registers.
Changes proc/gdbserial so that the 'g' and 'G' commands are never used
with debugserver since they seem to corrupt the thread state when used
on AVX-512 capable hardware.
Also changes TestClientServer_FpRegisters to be simpler and more
resilient to changes to the Go runtime.
Fixes#2479
Recent changes to the way registers are handled broke reporting of AVX
registers (i.e. YMMx). This change restores the functionality by:
- concatenating the higher half of the YMMx registers to their
corresponding XMMx lower half (YMMx registers do not have an
independent DWARF register number)
- modifying the formatSSEReg function to handle them when they are
present.
Fixes#2033
