Move wasm32-wasi toolchains off of fbcode-base

Summary:
D78500805 caused an issue using the toolchain on macosx, where the underlying `:fbcode-base` toolchain was failing to match on particular flags

```
Error running analysis for `fbcode//scripts/rust_foundation/arc_rust_fixtures/wasm:lints_wasm (cfg:dev-macos-x86_64-macosx-clang19-no-san#451711d0070bbb4d)`

Caused by:
    0: Error in configured node dependency, dependency chain follows (-> indicates depends on, ^ indicates same configuration as previous):
              fbcode//scripts/rust_foundation/arc_rust_fixtures/wasm:lints_wasm (cfg:dev-macos-x86_64-macosx-clang19-no-san#451711d0070bbb4d)
           -> fbcode//scripts/rust_foundation/arc_rust_fixtures/wasm:lints (ovr_config//platform/wasm:wasm32-wasip1#7e4dce05ea266e52)
           -> toolchains//:rust (^)
           -> fbcode//buck2/platform/toolchain:rust (^)
           -> fbcode//buck2/platform/rust:rust (^)
           -> fbsource//xplat/rust/toolchain/sysroot/1.88.0:std (^)
           -> fbsource//xplat/rust/toolchain/sysroot/1.88.0:std-0.0.0 (^)
           -> toolchains//:cxx (^)
           -> fbcode//buck2/platform/toolchain:cxx (^)
           -> fbcode//buck2/platform:cxx-wasm32-wasip1 (^)
           -> fbcode//buck2/platform:cxx-fbcode (^)
           -> fbcode//buck2/platform:cxx-fbcode-modeless (^)

    1: configuring attr `ldflags`
    2: None of 5 conditions matched configuration `ovr_config//platform/wasm:wasm32-wasip1#7e4dce05ea266e52` and no default was set:
         ovr_config//runtime/constraints:platform010
         ovr_config//runtime/constraints:platform010-aarch64
         ovr_config//runtime/constraints:platform010-aarch64-compat
         ovr_config//runtime/constraints:platform010-compat
         ovr_config//runtime/constraints:platform010-libcxx
```

Fix here is to move the toolchain entirely off of the `fbcode-base`, and maintain it standalone.

Reviewed By: dtolnay

Differential Revision: D78619980

fbshipit-source-id: 64b274280091a46dd73c8ab89b4b624bccc2e44c

Author: capickett

decls/cxx_rules.bzl

@@ -74,7 +74,7 @@ CxxTestType = ["gtest", "boost"]
 
 CxxToolProviderType = ["clang", "clang_cl", "clang_windows", "gcc", "windows", "windows_ml64"]
 
-LinkerProviderType = ["darwin", "gnu", "windows", "unknown"]
+LinkerProviderType = ["darwin", "gnu", "windows", "unknown", "wasm"]
 
 PicType = ["pic", "pdc"]