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>

.github/workflows/ports_qemu-arm.yml

@@ -19,14 +19,26 @@ concurrency:
   cancel-in-progress: true
 
 jobs:
-  build_and_test:
+  build_and_test_arm:
     runs-on: ubuntu-latest
     steps:
     - uses: actions/checkout@v4
     - name: Install packages
-      run: source tools/ci.sh && ci_qemu_arm_setup
+      run: source tools/ci.sh && ci_qemu_setup_arm
     - name: Build and run test suite
-      run: source tools/ci.sh && ci_qemu_arm_build
+      run: source tools/ci.sh && ci_qemu_build_arm
+    - name: Print failures
+      if: failure()
+      run: tests/run-tests.py --print-failures
+
+  build_and_test_rv32:
+    runs-on: ubuntu-latest
+    steps:
+    - uses: actions/checkout@v4
+    - name: Install packages
+      run: source tools/ci.sh && ci_qemu_setup_rv32
+    - name: Build and run test suite
+      run: source tools/ci.sh && ci_qemu_build_rv32
     - name: Print failures
       if: failure()
       run: tests/run-tests.py --print-failures

.github/workflows/ports_qemu-riscv.yml

@@ -1,33 +0,0 @@
-name: qemu-riscv port
-
-on:
-  push:
-  pull_request:
-    paths:
-      - '.github/workflows/*.yml'
-      - 'tools/**'
-      - 'py/**'
-      - 'extmod/**'
-      - 'shared/**'
-      - 'lib/**'
-      - 'drivers/**'
-      - 'ports/qemu-arm/main.c'
-      - 'ports/qemu-riscv/**'
-      - 'tests/**'
-
-concurrency:
-  group: ${{ github.workflow }}-${{ github.ref }}
-  cancel-in-progress: true
-
-jobs:
-  build_and_test:
-    runs-on: ubuntu-latest
-    steps:
-    - uses: actions/checkout@v4
-    - name: Install packages
-      run: source tools/ci.sh && ci_qemu_riscv_setup
-    - name: Build and run test suite
-      run: source tools/ci.sh && ci_qemu_riscv_build
-    - name: Print failures
-      if: failure()
-      run: grep --before-context=100 --text "FAIL" ports/qemu-riscv/build/console.out