Fix emar invocation on Windows

On Windows we hit the command line length limit when building the wasm
port of Qt. We must not pass the list of object files directly to emar,
but write a response file which is then passed via the @ parameter.

Fixes: QTBUG-75257
Change-Id: Id518fd81325716b8efaba29f495568a9a3e34be4
Reviewed-by: Kai Koehne <kai.koehne@qt.io>
Reviewed-by: Edward Welbourne <edward.welbourne@qt.io>
Reviewed-by: Lorn Potter <lorn.potter@gmail.com>

mkspecs/wasm-emscripten/qmake.conf

@@ -94,6 +94,12 @@ QMAKE_PREFIX_STATICLIB  = lib
 QMAKE_EXTENSION_STATICLIB = a      # llvm bitcode
 
 QMAKE_AR                = emar cqs
+equals(QMAKE_HOST.os, Windows) {
+    QMAKE_AR_CMD = \
+        "$(file >$(OBJECTS_DIR)/$(TARGET).rsp, $(OBJECTS))$$escape_expand(\\n\\t)" \
+        "$(AR) $(DESTDIR)$(TARGET) @$(OBJECTS_DIR)/$(TARGET).rsp"
+}
+
 QMAKE_DISTCLEAN            += *.html *.js *.wasm
 
 load(qt_config)