The offset of state component i can be found via
CPUID.(EAX=0DH,ECX=i):EBX. The ZMM_Hi256 is state component 6, so we use
CPUID to enumerate the offset instead of hardcoding.
For core dumps, we guess the ZMM_Hi256 offset based on xcr0 and the
length of xsave region. The logic comes from binutils-gdb.
Fixes#3827.
Delve represents registerized variables (fully or partially) using
compositeMemory, implementing proc.(*compositeMemory).WriteMemory is
necessary to make SetVariable and function calls work when Go will
switch to using the register calling convention in 1.17.
This commit also makes some refactoring by moving the code that
converts between register numbers and register names out of pkg/proc
into a different package.
