riscv: Separate FPU register size from core register size in gdbstub [v2]

riscv: Separate FPU register size from core register size in gdbstub [v2]

The size of the FPU registers is dictated by the 'f' and 'd' features,
not the core processor register size. Processors with the 'd' feature
have 64-bit FPU registers. Processors without the 'd' feature but with
the 'f' feature have 32-bit FPU registers.

Signed-off-by: Keith Packard <keithp@keithp.com>
[Palmer: This requires manually triggering a rebuild of
riscv32-softmmu/gdbstub-xml.c]
Signed-off-by: Palmer Dabbelt <palmerdabbelt@google.com>