feat: Rust std target + ruststd tests; SYS_EXIT_GROUP; fix mkdir/create under missing parent

programs/rust/.cargo/config.toml

@@ -1,13 +1,7 @@
-# Phase 1 Rust support: no_std + alloc on a stock nightly, built with
-# -Zbuild-std against the custom target spec next to this directory.
 # Paths below are relative to programs/rust/ (the parent of .cargo/).
 
 [build]
-target = "x86_64-unknown-montauk.json"
-
-[unstable]
-build-std = ["core", "alloc"]
-json-target-spec = true
+target = "x86_64-unknown-montauk"
 
 [target.x86_64-unknown-montauk]
 # The montauk GCC driver supplies crt1/crti/crtbegin, -lc and libgcc, and