bazel/wasm: upgrade wasmtime from 32.0.0 to 42.0.2

Additional changes required by the upgrade:

- Bump Rust toolchain from 1.86.0 to 1.91.0 (wasmtime 42 MSRV)
- Add PULLEY and ALL_ARCH feature flags to wasmtime.BUILD conf.h
  template substitutions (new in wasmtime 42)
- Remove wasmtime_config_async_support_set() call: async support is
  now implicit, the separate toggle was deprecated and removed from
  the C API (bytecodealliance/wasmtime#12371)
- Bump rules_rust 0.60.0 -> 0.70.0 and opt in to
  experimental_use_allocator_libraries_with_mangled_symbols so the
  mangled __rustc::* allocator shim symbols (introduced in rustc
  1.87+) are provided when a Rust staticlib is linked into a
  cc_binary via cc_common.link. Without this flag the link fails
  with undefined references to __rustc::__rust_alloc,
  __rustc::__rust_dealloc, etc. Background:
    - Issue:   bazelbuild/rules_rust#3459
    - Fix PR:  bazelbuild/rules_rust#3403
    - Landed in rules_rust 0.63.0 as an opt-in setting.
- Mirror the rustc tarball through vectorized-public to reduce
  dependence on static.rust-lang.org.
- Incidental: bump bazel_skylib 1.8.1 -> 1.8.2 and rules_shell
  0.4.1 -> 0.6.1 to silence direct-dependency version warnings
  pulled in by the new rules_rust.

Author: travisdowns

.bazelrc

@@ -35,6 +35,20 @@ build --@rules_go//go/config:pure
 build:gofips --//bazel:gofips=True --@rules_go//go/toolchain:sdk_name=go_sdk_with_systemcrypto
 build:gofips --@rules_go//go/config:pure=false
 
+#################
+# Rust settings #
+#################
+
+# Use rules_rust's Rust-side allocator_library that defines the
+# mangled `__rustc::*` allocator shim symbols (rust_alloc,
+# rust_dealloc, etc.). The default cc_allocator_library predates the
+# allocator-symbol mangling in rustc 1.87+ and leaves those symbols
+# undefined when a Rust staticlib is linked into a cc_binary via
+# cc_common.link. See bazelbuild/rules_rust#3459.
+build --@rules_rust//rust/settings:experimental_use_allocator_libraries_with_mangled_symbols=True
+
+# end of rust settings #
+
 # Improve caching and readability of filepaths that the compiler uses by not using
 # absolute paths, and instead use paths that are relative to the redpanda repo.
 # https://github.com/bazel-contrib/toolchains_llvm/pull/445#issuecomment-2605443516

MODULE.bazel

@@ -350,7 +350,7 @@ rust.toolchain(
         "aarch64-unknown-linux-gnu",
         "x86_64-unknown-linux-gnu",
     ],
-    versions = ["1.86.0"],
+    versions = ["1.91.0"],
 )
 use_repo(rust, "rust_toolchains")