Merge #79

79: update to riscv 0.7 r=almindor a=smsxgli

hi everyone, I am a new guy with both github, rust and riscv, and this is my first PR, so if I miss something or do something wrong, please let me know (and forgive my poor english, since I am not a native speaker).
crate `riscv` v0.7 solved link error about [`different hardware float abi`](https://github.com/rust-embedded/riscv/issues/85

), but riscv-rt still depend on `riscv` v0.6.

Co-authored-by: smsxgli <smsxgli@gmail.com>