plugins: add riscv into kernel plugin for build dtbs

[AristoChen]

Hi,

I was using kernel plugin to build a RISC-V kernel, but it won't build dtbs, and I found it will append dtbs only if ARCH matches arm, arm64 or riscv64, however, the ARCH in my case is riscv. The following are some logs

Before the code change:

2023-07-01 23:03:45.814 :: Building kernel...
2023-07-01 23:03:45.814 :: ++ nproc
2023-07-01 23:03:45.816 :: + make -j8 -C /home/vagrant/aristo/UbuntuCoreOS/cache/starfive_visionfive2_backup/kernel_snap/parts/kernel/src O=/home/vagrant/aristo/UbuntuCoreOS/cache/starfive_visionfive2_backup/kernel_snap/parts/kernel/build ARCH=riscv CROSS_COMPILE=riscv64-linux-gnu- Image modules

After the code change:

2023-07-01 23:27:34.931 :: Building kernel...
2023-07-01 23:27:34.931 :: ++ nproc
2023-07-01 23:27:34.932 :: + make -j8 -C /home/vagrant/aristo/UbuntuCoreOS/cache/starfive_visionfive2_backup/kernel_snap/parts/kernel/src O=/home/vagrant/aristo/UbuntuCoreOS/cache/starfive_visionfive2_backup/kernel_snap/parts/kernel/build ARCH=riscv CROSS_COMPILE=riscv64-linux-gnu- Image modules dtbs

• Have you followed the guidelines for contributing?
• Have you signed the CLA?
• Have you successfully run make lint?
• [] Have you successfully run pytest tests/unit?

---

[codecov-commenter]

Codecov Report

Merging #4252 (1447c9b) into main (ab2c135) will decrease coverage by 0.02%.
The diff coverage is 100.00%.

❗ Your organization is not using the GitHub App Integration. As a result you may experience degraded service beginning May 15th. Please install the Github App Integration for your organization. Read more.

@@            Coverage Diff             @@
##             main    #4252      +/-   ##
==========================================
- Coverage   88.98%   88.96%   -0.02%     
==========================================
  Files         296      295       -1     
  Lines       20192    20151      -41     
==========================================
- Hits        17968    17928      -40     
+ Misses       2224     2223       -1     

Impacted Files	Coverage Δ
snapcraft/parts/plugins/kernel_plugin.py	100.00% <100.00%> (ø)

... and 3 files with indirect coverage changes

📣 We’re building smart automated test selection to slash your CI/CD build times. Learn more