wasm: use precise GC for WebAssembly (including WASI)

With a few small modifications, all the problems with `-gc=precise` in
WebAssembly seem to have been fixed.

I didn't do any performance measurements, but this is supposed to
improve GC performance.

targets/wasip1.json

@@ -8,6 +8,7 @@
 	"linker":        "wasm-ld",
 	"libc":          "wasi-libc",
 	"rtlib":         "compiler-rt",
+	"gc":            "precise",
 	"scheduler":     "asyncify",
 	"default-stack-size": 65536,
 	"cflags": [

targets/wasip2.json

@@ -9,6 +9,7 @@
 	"linker":        "wasm-ld",
 	"libc":          "wasmbuiltins",
 	"rtlib":         "compiler-rt",
+	"gc":            "precise",
 	"scheduler":     "asyncify",
 	"default-stack-size": 65536,
 	"cflags": [

targets/wasm.json

@@ -8,6 +8,7 @@
 	"linker":        "wasm-ld",
 	"libc":          "wasi-libc",
 	"rtlib":         "compiler-rt",
+	"gc":            "precise",
 	"scheduler":     "asyncify",
 	"default-stack-size": 65536,
 	"cflags": [