Add MAP_ANON support for mmap/munmap for standalone Wasm

[bryanmcquade]

Sam Clegg asked me to open an issue to extend the current support for MAP_ANON that was added in

emscripten/src/library_syscall.js

Lines 240 to 247 in 22b499d

	// MAP_ANONYMOUS (aka MAP_ANON) isn't actually defined by POSIX spec,
	// but it is widely used way to allocate memory pages on Linux, BSD and Mac.
	// In this case fd argument is ignored.
	if ((flags & {{{ cDefine('MAP_ANONYMOUS') }}}) !== 0) {
	ptr = mmapAlloc(len);
	if (!ptr) return -{{{ cDefine('ENOMEM') }}};
	allocated = true;
	} else {

to also work for standalone wasm.

The motivation for this is that I (and others) use the abseil library in standalone wasm, and abseil depends on mmap availability in one of its low level routines, used by things like lock acquires in debug builds (among other cases), which are widely depended on by various features across abseil. The specific abseil code that calls mmap is here: https://github.com/abseil/abseil-cpp/blob/1ae9b71c474628d60eb251a3f62967fe64151bb2/absl/base/internal/low_level_alloc.cc#L562

[bryanmcquade]

#13312 also requested the ability to provide an mmap/munmap implementation for standalone wasm, so this seems like it would be useful for other use cases as well.