wasm: add Boehm GC support

This adds support for `-gc=boehm` on `-target=wasip1` and `-target=wasm`
(in a browser or NodeJS). Notably it does *not* add Boehm GC support for
`-target=wasip2`, since that target doesn't have a real libc.

targets/wasip1.json

@@ -23,7 +23,8 @@
 		"--no-demangle"
 	],
 	"extra-files": [
-		"src/runtime/asm_tinygowasm.S"
+		"src/runtime/asm_tinygowasm.S",
+		"src/runtime/gc_boehm.c"
 	],
 	"emulator":      "wasmtime run --dir={tmpDir}::/tmp {}"
 }

targets/wasm.json

@@ -24,7 +24,8 @@
 		"--no-demangle"
 	],
 	"extra-files": [
-		"src/runtime/asm_tinygowasm.S"
+		"src/runtime/asm_tinygowasm.S",
+		"src/runtime/gc_boehm.c"
 	],
 	"emulator":      "node {root}/targets/wasm_exec.js {}"
 }