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.

src/runtime/gc_boehm.c

@@ -0,0 +1,37 @@
+//go:build none
+
+// This file is included in the build on systems that support the Boehm GC,
+// despite the //go:build line above.
+
+#include <stdint.h>
+
+typedef void (* GC_push_other_roots_proc)(void);
+void GC_set_push_other_roots(GC_push_other_roots_proc);
+
+typedef void(* GC_warn_proc)(const char *msg, uintptr_t arg);
+void GC_set_warn_proc(GC_warn_proc p);
+
+void tinygo_runtime_bdwgc_callback(void);
+
+static void callback(void) {
+    tinygo_runtime_bdwgc_callback();
+}
+
+static void warn_proc(const char *msg, uintptr_t arg) {
+}
+
+void tinygo_runtime_bdwgc_init(void) {
+    GC_set_push_other_roots(callback);
+#if defined(__wasm__)
+    // There are a lot of warnings on WebAssembly in the form:
+    //
+    //     GC Warning: Repeated allocation of very large block (appr. size 68 KiB):
+    //         May lead to memory leak and poor performance
+    //
+    // The usual advice is to use something like GC_malloc_ignore_off_page but
+    // unfortunately for most allocations that's not allowed: Go allocations can
+    // legitimately hold pointers further than one page in the allocation. So
+    // instead we just disable the warning.
+    GC_set_warn_proc(warn_proc);
+#endif
+}