wasip2: build against wasi-libc instead of the Go mini-libc

Wasip2 previously got its libc from a ~1300-line hand-written Go
implementation (src/syscall/libc_wasip2.go) calling wasi:cli/filesystem/io
component bindings directly. wasi-libc upstream already ships a C
implementation of the same wasip2 surface (wasip2.c, wasip2_stdio.c,
descriptor_table.c, socket support), generated from the same WIT
interfaces, so link wasip2 against that instead.

- builder/wasilibc.go: turn libWasiLibc into a newWasiLibc(preview, name)
  factory. libWasiLibcWasip2 builds with __wasip2__ defined and the
  descriptor-table-based socket/stdio sources instead of wasip1's
  cloudlibc syscalls. Distinct library name since the build cache is
  keyed by name+triple and both targets share the same wasm32 triple.
- builder/build.go / targets/wasip2.json: route the "wasi-libc" case to
  the wasip2 variant when the wasip2 build tag is set.
- src/syscall/syscall_libc.go and syscall_libc_wasi.go were already
  shared between wasip1/wasip2 (linker-resolved //export symbols), so
  they needed no changes. env_libc.go/errno_wasilibc.go are now shared
  too; the wasip2-only mini-libc files are deleted.
- Environ() needed to move to wasip1/wasip2-specific files
  (environ_libc.go / environ_wasip2.go): wasi-libc's eager,
  constructor-based environ population is fine for wasip1 (a plain core
  wasm module) but traps under wasip2's component model when it fires
  reentrantly during cabi_realloc ("cannot leave component instance").
  wasip2 uses wasi-libc's documented lazy __wasilibc_get_environ()
  accessor instead.
- runtime_wasip2.go: cabi_realloc now goes through the malloc-tracked
  libc_realloc instead of the GC-internal realloc, since wasi-libc's C
  code (e.g. wasip2_string_free) frees buffers allocated during
  component-ABI lifting with a plain free().
- lib/wasi-libc-wasip2-stub.c: satisfies wasip2.c's forced-link
  reference to wasi-libc's own crt1 component-type metadata, which
  TinyGo doesn't build (it provides its own entry points and its own
  component-embedding step).

Co-Authored-By: Claude Sonnet 5 <noreply@anthropic.com>

Author: dgryski

builder/build.go

@@ -167,7 +167,11 @@ func Build(pkgName, outpath, tmpdir string, config *compileopts.Config) (BuildRe
 		defer unlock()
 		libcDependencies = append(libcDependencies, libcJob)
 	case "wasi-libc":
-		libcJob, unlock, err := libWasiLibc.load(config, tmpdir)
+		lib := libWasiLibc
+		if slices.Contains(config.BuildTags(), "wasip2") {
+			lib = libWasiLibcWasip2
+		}
+		libcJob, unlock, err := lib.load(config, tmpdir)
 		if err != nil {
 			return BuildResult{}, err
 		}