qemu-arm: Merge RISC-V 32-bit support into qemu-arm port.

Currently both the qemu-arm and qemu-riscv ports share a lot of code and
functionality.  This commit merges the qemu-riscv port into the qemu-arm
port.  The only real differences between the two are the toolchains used to
build the code, and the initialisation/startup framework.  Everything else
is pretty much the same, so this brings the following benefits:
- less code duplication
- less burden on maintenance
- generalised qemu port, could in the future support other architectures

A new board `VIRT_RV32` has been added to the qemu-arm port which is the
existing RISC-V board from the qemu-riscv port.  To build it:

    $ make BOARD=VIRT_RV32 repl

To cleanly separate the code for the different architectures, startup code
has been moved to ports/qemu-arm/mcu/<arch>/.

Signed-off-by: Damien George <damien@micropython.org>

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