util: exclude mmap-alloc.c from compilation target on Emscripten

util: exclude mmap-alloc.c from compilation target on Emscripten

Emscripten does not support partial unmapping of mmapped memory
regions[1]. This limitation prevents correct implementation of qemu_ram_mmap
and qemu_ram_munmap, which rely on partial unmap behavior.

As a workaround, this commit excludes mmap-alloc.c from the Emscripten
build. Instead, for Emscripten build, this modifies qemu_anon_ram_alloc to
use qemu_memalign in place of qemu_ram_mmap, and disable memory backends
that rely on mmap, such as memory-backend-file and memory-backend-shm.

[1] https://github.com/emscripten-core/emscripten/blob/d4a74336f23214bf3304d9eb0d03966786b30a36/system/lib/libc/emscripten_mmap.c#L61

Signed-off-by: Kohei Tokunaga <ktokunaga.mail@gmail.com>
Link: https://lore.kernel.org/r/76834f933ee4f14eeb5289d21c59d306886e58e9.1745820062.git.ktokunaga.mail@gmail.com
Signed-off-by: Paolo Bonzini <pbonzini@redhat.com>