build: ensure toolchain is passed to bundled deps #37587

Problem:
When building Neovim with Emscripten for WebAssembly, the
`CMAKE_TOOLCHAIN_FILE` is not propagated to dependency builds.
So libuv attempts to include Linux-specific headers like
`sys/epoll.h` which don't exist in the WebAssembly environment.

From #37572:

    …/NeovimWeb/external/neovim/.deps/build/src/libuv/src/unix/linux.c:44:10:
    fatal error: 'sys/epoll.h' file not found
       44 | #include <sys/epoll.h> |          ^~~~~~~~~~~~~

Solution:
1. Add initialization for DEPS_CMAKE_ARGS to prevent CMake errors.
2. Implement conditional logic to always pass CMAKE_TOOLCHAIN_FILE to
   dependencies.

This allows dependencies like libuv to detect they're building for
WebAssembly and disable incompatible Linux specific features such as
epoll which resolves the sys/epoll.h error.

cmake.deps/CMakeLists.txt

@@ -62,6 +62,11 @@ option(USE_EXISTING_SRC_DIR "Skip download of deps sources in case of existing s
 
 set_default_buildtype(Release)
 get_property(isMultiConfig GLOBAL PROPERTY GENERATOR_IS_MULTI_CONFIG)
+
+if (NOT DEFINED DEPS_CMAKE_ARGS) 
+  set(DEPS_CMAKE_ARGS) 
+endif() 
+
 if(NOT isMultiConfig)
   list(APPEND DEPS_CMAKE_ARGS -D CMAKE_BUILD_TYPE=${CMAKE_BUILD_TYPE})
 endif()
@@ -73,6 +78,13 @@ if(HAS_OG_FLAG)
   set(DEFAULT_MAKE_CFLAGS CFLAGS+=-Og ${DEFAULT_MAKE_CFLAGS})
 endif()
 
+# Always pass the toolchain if it exists
+if(CMAKE_TOOLCHAIN_FILE)
+  list(APPEND DEPS_CMAKE_ARGS
+    -D CMAKE_TOOLCHAIN_FILE=${CMAKE_TOOLCHAIN_FILE}
+  )
+endif()
+
 set(DEPS_INCLUDE_FLAGS "-I\"${DEPS_INSTALL_DIR}/include\" -I\"${DEPS_INSTALL_DIR}/include/luajit-2.1\"")
 
 # If the macOS deployment target is not set manually (via $MACOSX_DEPLOYMENT_TARGET),