Add support for reading a macho corefile with CPU_TYPE_RISCV and the
riscv32 general purpose register file. I added code for the floating
point and exception registers too, but haven't exercised this. If we
start putting the full CSR register bank in a riscv corefile, it'll be
in separate 4k byte chunks, but I don't have a corefile to test against
that so I haven't written the code to read it.
The RegisterContextDarwin_riscv32 is copied & in the style of the other
RegisterContextDarwin classes; it's not the first choice I would make
for representing this, but it wasn't worth changing for this cputype.
rdar://145014653
096ab51de0