Add wasm32-wasi-musl target

I would like to be able to compile wasm/wasi with zig + bazel. This adds
a toolchain to be able to do so. Wasm/wasi is a pretty rapidly changing
space, but I've choosen the most specific naming that I could (WASI
preview 1).

I've confirmed this works by running the compiled binary with wasmtime,
a popular wasm/wasi runtime.

```shell
bazel run --run_under=wasmtime //test/c:which_libc_wasip1_wasm
```

Signed-off-by: Tyler Rockwood <rockwood@redpanda.com>

MODULE.bazel

@@ -46,4 +46,6 @@ register_toolchains(
     # arm64 toolchains for libc-aware platforms:
     "@zig_sdk//libc_aware/toolchain:linux_arm64_gnu.2.28",
     "@zig_sdk//libc_aware/toolchain:linux_arm64_musl",
+    # wasm/wasi toolchains
+    "@zig_sdk//toolchain:wasip1_wasm",
 )

test/c/BUILD

@@ -53,3 +53,10 @@ cc_binary(
         ("windows_arm64", "//platform:windows_arm64", []),
     ]
 ]
+
+platform_binary(
+    name = "which_libc_wasip1_wasm",
+    src = "which_libc",
+    platform = "//platform:wasip1_wasm",
+    tags = [],
+)

test/c/main.c

@@ -10,6 +10,9 @@
 #elif __linux__
 #include <features.h>
 #define OS "linux"
+#elif __wasi__
+#include <features.h>
+#define OS "wasi"
 #else
 #   error "Unknown compiler!"
 #endif

toolchain/platform/defs.bzl

@@ -13,6 +13,8 @@ def declare_platforms():
         for bzlos, oss in _OS.items():
             for os in oss:
                 declare_platform(gocpu, zigcpu, bzlos, os)
+    # We can support GOARCH=wasm32 after https://github.com/golang/go/issues/63131
+    declare_platform("wasm", "wasm32", "wasi", "wasip1")
 
 def declare_libc_aware_platforms():
     # create @zig_sdk//{os}_{arch}_platform entries with zig and go conventions

toolchain/private/defs.bzl

@@ -48,6 +48,7 @@ def target_structs():
         ret.append(_target_linux_musl(gocpu, zigcpu))
         for glibc in _GLIBCS:
             ret.append(_target_linux_gnu(gocpu, zigcpu, glibc))
+    ret.append(_target_wasm())
     return ret
 
 def _target_macos(gocpu, zigcpu):
@@ -199,3 +200,25 @@ def _target_linux_musl(gocpu, zigcpu):
         ld_zig_subcmd = "ld.lld",
         artifact_name_patterns = [],
     )
+
+def _target_wasm():
+    return struct(
+        gotarget = "wasip1_wasm",
+        zigtarget = "wasm32-wasi-musl",
+        includes = [
+          "libc/include/wasm-wasi-musl",
+          "libc/wasi",
+        ] + _INCLUDE_TAIL,
+        linkopts = [],
+        dynamic_library_linkopts = [],
+        supports_dynamic_linker = True,
+        copts = [],
+        libc = "musl",
+        bazel_target_cpu = "wasm32",
+        constraint_values = [
+            "@platforms//os:wasi",
+            "@platforms//cpu:wasm32",
+        ],
+        ld_zig_subcmd = "ld.lld",
+        artifact_name_patterns = [],
+    )

toolchain/zig-wrapper.zig

@@ -279,11 +279,11 @@ fn getRunMode(self_exe: []const u8, self_base_noexe: []const u8) error{BadParent
     var it = mem.split(u8, triple, "-");
 
     const arch = it.next() orelse return error.BadParent;
-    if (mem.indexOf(u8, "aarch64,x86_64", arch) == null)
+    if (mem.indexOf(u8, "aarch64,x86_64,wasm32", arch) == null)
         return error.BadParent;
 
     const got_os = it.next() orelse return error.BadParent;
-    if (mem.indexOf(u8, "linux,macos,windows", got_os) == null)
+    if (mem.indexOf(u8, "linux,macos,windows,wasi", got_os) == null)
         return error.BadParent;
 
     // ABI triple is too much of a moving target