Switch to hpm-riscv-rt

[andelf]

As planned. The hpm-riscv-rt crate is now ready for use: https://github.com/hpmicro-rs/hpm-riscv-rt

Major differences vs riscv-rt:

• Use Andes V5's vectored interrupt
• Fits for HPMicro's 32-bit RISC-V cores: memory split, etc.
• Work with hpm-metapac (interrupt tables)