Auto merge of #3023 - RalfJung:gamma, r=RalfJung

add gamma function shims

needs a rustup to fetch the new declarations in std

Author: bors

crates/ide-completion/src/completions/extern_abi.rs

@@ -30,6 +30,8 @@ const SUPPORTED_CALLING_CONVENTIONS: &[&str] = &[
     "efiapi",
     "avr-interrupt",
     "avr-non-blocking-interrupt",
+    "riscv-interrupt-m",
+    "riscv-interrupt-s",
     "C-cmse-nonsecure-call",
     "wasm",
     "system",