MemoryAccessor.zig: Make it build with 'other' target OS

Make the `have_msync` check depend directly on the presence of
posix.system.msync (instead of indirectly via a list of systems that may
or may not have msync).

Add an explicit empty `msync` symbol in the default posix.system struct
and in the Wasi system.

Author: rootbeer

lib/std/c.zig

@@ -9069,6 +9069,7 @@ pub const gettimeofday = switch (native_os) {
 
 pub const msync = switch (native_os) {
     .netbsd => private.__msync13,
+    .wasi => {},
     else => private.msync,
 };
 

lib/std/debug/MemoryAccessor.zig

@@ -78,9 +78,6 @@ pub fn load(ma: *MemoryAccessor, comptime Type: type, address: usize) ?Type {
 }
 
 pub fn isValidMemory(address: usize) bool {
-    // We are unable to determine validity of memory for freestanding targets
-    if (native_os == .freestanding or native_os == .uefi) return true;
-
     const aligned_address = address & ~@as(usize, @intCast((page_size - 1)));
     if (aligned_address == 0) return false;
     const aligned_memory = @as([*]align(page_size) u8, @ptrFromInt(aligned_address))[0..page_size];
@@ -122,7 +119,4 @@ pub fn isValidMemory(address: usize) bool {
     }
 }
 
-const have_msync = switch (native_os) {
-    .wasi, .emscripten, .windows => false,
-    else => true,
-};
+const have_msync = @TypeOf(posix.system.msync) != void;

lib/std/posix.zig

@@ -52,6 +52,7 @@ else switch (native_os) {
         pub const fd_t = void;
         pub const uid_t = void;
         pub const gid_t = void;
+        pub const msync = {};
     },
 };