target/riscv: accessors to registers upper part and 128-bit load/store

Get function to retrieve the 64 top bits of a register, stored in the gprh
field of the cpu state. Set function that writes the 128-bit value at once.
The access to the gprh field can not be protected at compile time to make
sure it is accessed only in the 128-bit version of the processor because we
have no way to indicate that the misa_mxl_max field is const.

The 128-bit ISA adds ldu, lq and sq. We provide support for these
instructions. Note that (a) we compute only 64-bit addresses to actually
access memory, cowardly utilizing the existing address translation mechanism
of QEMU, and (b) we assume for now little-endian memory accesses.

Signed-off-by: Frédéric Pétrot <frederic.petrot@univ-grenoble-alpes.fr>
Co-authored-by: Fabien Portas <fabien.portas@grenoble-inp.org>
Reviewed-by: Alistair Francis <alistair.francis@wdc.com>
Message-id: 20220106210108.138226-10-frederic.petrot@univ-grenoble-alpes.fr
Signed-off-by: Alistair Francis <alistair.francis@wdc.com>

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