third-party/rust: update riscv with PR 198

Summary: Add a couple of helper methods rust-embedded/riscv#198

Reviewed By: dreiss

Differential Revision:
D55329981

Privacy Context Container: L1122763

fbshipit-source-id: fa32db2553ae5caa3118312d038db734d079bc4d

Cargo.toml

@@ -11,6 +11,7 @@ lru-disk-cache = { git = "https://github.com/mozilla/sccache", rev = "fdbf843d17
 perf-event = { version = "0.4", git = "https://github.com/krallin/perf-event.git", rev = "86224a9bc025d5d19f719542f27c8c629a08b167" }
 perf-event-open-sys = { version = "4.0", git = "https://github.com/krallin/perf-event.git", rev = "86224a9bc025d5d19f719542f27c8c629a08b167" }
 quickcheck = { git = "https://github.com/jakoschiko/quickcheck", rev = "6ecdf5bb4b0132ce66670b4d46453aa022ea892c" }
+riscv = { git = "https://github.com/rust-embedded/riscv.git", rev = "6f0e9da1ee836179fcb8d481eebce0d4e7cb864d" }
 
 [workspace]
 members = [