Use mmap std.heap.page_allocator impl when compiling for Wasm + libc

When linking libc, it should be the libc that manages the heap. The main
Wasm memory might have been configured as non-growable, which makes
`WasmAllocator` a poor default and causes the common `DebugAllocator`
use case fail with OOM errors unless the user uses `std_options` to
override the default page allocator. Additionally, on Emscripten,
growing Wasm memory without notifying the JS glue code will cause array
buffers to get detached and lead to spurious crashes.

Author: castholm

lib/std/c.zig

@@ -1811,7 +1811,16 @@ pub const POLL = switch (native_os) {
 /// Basic memory protection flags
 pub const PROT = switch (native_os) {
     .linux => linux.PROT,
-    .emscripten => emscripten.PROT,
+    // https://github.com/emscripten-core/emscripten/blob/08e2de1031913e4ba7963b1c56f35f036a7d4d56/system/lib/libc/musl/include/sys/mman.h#L57-L62
+    // lib/libc/include/wasm-wasi-musl/sys/mman.h
+    .emscripten, .wasi => struct {
+        pub const NONE = 0;
+        pub const READ = 1;
+        pub const WRITE = 2;
+        pub const EXEC = 4;
+        pub const GROWSDOWN = 0x01000000;
+        pub const GROWSUP = 0x02000000;
+    },
     // https://github.com/SerenityOS/serenity/blob/6d59d4d3d9e76e39112842ec487840828f1c9bfe/Kernel/API/POSIX/sys/mman.h#L28-L31
     .openbsd, .haiku, .dragonfly, .netbsd, .illumos, .freebsd, .windows, .serenity => struct {
         /// page can not be accessed
@@ -8647,7 +8656,9 @@ pub const O = switch (native_os) {
 
 pub const MAP = switch (native_os) {
     .linux => linux.MAP,
-    .emscripten => packed struct(u32) {
+    // https://github.com/emscripten-core/emscripten/blob/08e2de1031913e4ba7963b1c56f35f036a7d4d56/system/lib/libc/musl/include/sys/mman.h#L21-L39
+    // lib/libc/include/wasm-wasi-musl/sys/mman.h
+    .emscripten, .wasi => packed struct(u32) {
         TYPE: enum(u4) {
             SHARED = 0x01,
             PRIVATE = 0x02,

lib/std/heap.zig

@@ -356,7 +356,7 @@ pub const page_allocator: Allocator = if (@hasDecl(root, "os") and
     @hasDecl(root.os, "heap") and
     @hasDecl(root.os.heap, "page_allocator"))
     root.os.heap.page_allocator
-else if (builtin.target.cpu.arch.isWasm()) .{
+else if (builtin.target.cpu.arch.isWasm() and !builtin.link_libc) .{
     .ptr = undefined,
     .vtable = &WasmAllocator.vtable,
 } else if (builtin.target.os.tag == .plan9) .{

lib/std/os/emscripten.zig

@@ -331,15 +331,6 @@ pub const POLL = struct {
     pub const RDBAND = 0x080;
 };
 
-pub const PROT = struct {
-    pub const NONE = 0x0;
-    pub const READ = 0x1;
-    pub const WRITE = 0x2;
-    pub const EXEC = 0x4;
-    pub const GROWSDOWN = 0x01000000;
-    pub const GROWSUP = 0x02000000;
-};
-
 pub const rlim_t = u64;
 
 pub const RLIM = struct {