chore(deps): upgrade the custom fork of riscv-rt

Fixes the build failures on the latest nightly compiler.

    error[E0658]: const operands for inline assembly are unstable
       --> .../.cargo/git/checkouts/rust-riscv-rt-6ba025082ec0749f/cd7b460/src/lib.rs:513:9
        |
    513 |         HAS_MUL = const cfg!(target_feature = "m") as u32,
        |         ^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^
        |

Author: yvt

Cargo.lock

@@ -43,7 +43,7 @@ branch = "0.5-asm"
 # the precompiled object file with naked functions, which will always be
 # complied with the expected floating-point ABI.
 git = "https://github.com/yvt/rust-riscv-rt.git"
-rev = "cd7b460d1940630d235b5a7b46cc50421c12e39b"
+rev = "07ade94990ef49b35bf298f9cbe59c671c6f1101"
 version = "0.6.2"
 
 [profile.release]