Add WASM support via libm crate fallback

- m_sys.rs: Keep only FFI declarations for unix/windows
- m.rs: Add fallback to libm crate for WASM and other non-native targets
- Cargo.toml: Add libm dependency for non-unix/windows targets

The fallback uses pure Rust libm crate when native libm is unavailable
(e.g., WASM builds), avoiding 'env' import errors.

Author: youknowone

Cargo.toml

@@ -22,6 +22,10 @@ _bigint = ["dep:num-traits", "dep:num-integer"]  # Internal feature. User must u
 # See also: https://github.com/python/cpython/issues/132763
 mul_add = []
 
+# For WASM and other targets without native libm
+[target.'cfg(not(any(unix, windows)))'.dependencies]
+libm = "0.2"
+
 [dependencies]
 libc = "0.2"
 num-complex = { version = "0.4", optional = true }