Fix linking SIDE_MODULE with ports (#26514)

With the removal of the RELOCATABLE flag
(cfb4642),
building side module with ports are not working correctly because of 1)
`-fPIC` flag missing when building ports, and 2) `pic` directory not
correctly passed when calculating the libdir.

Without this, building a side module that links to the port library
fails with the following error:

```c
// dummy.c
#include <emscripten/emscripten.h>
#include <jpeglib.h>

EMSCRIPTEN_KEEPALIVE
int dummy_libjpeg_smoke_test(void) {
	struct jpeg_decompress_struct cinfo;
	struct jpeg_error_mgr jerr;

	cinfo.err = jpeg_std_error(&jerr);
	jpeg_create_decompress(&cinfo);
	jpeg_destroy_decompress(&cinfo);

	return 0;
}

```

```
$ emcc dummy.c -o dummy.wasm -s USE_LIBJPEG=1 -s SIDE_MODULE=1 -ljpeg

wasm-ld: error: /home/siha/dev/temp/emscripten-regression/emsdk/upstream/emscripten/cache/sysroot/lib/wasm32-emscripten/libjpeg.a(jdinput.c.o): relocation R_WASM_TABLE_INDEX_SLEB cannot be used against symbol `finish_input_pass`; recompile with -fPIC
wasm-ld: error: /home/siha/dev/temp/emscripten-regression/emsdk/upstream/emscripten/cache/sysroot/lib/wasm32-emscripten/libjpeg.a(jdinput.c.o): relocation R_WASM_TABLE_INDEX_SLEB cannot be used against symbol `start_input_pass`; recompile with -fPIC
wasm-ld: error: /home/siha/dev/temp/emscripten-regression/emsdk/upstream/emscripten/cache/sysroot/lib/wasm32-emscripten/libjpeg.a(jdinput.c.o): relocation R_WASM_TABLE_INDEX_SLEB cannot be used against symbol `reset_input_controller`; recompile with -fPIC
wasm-ld: error: /home/siha/dev/temp/emscripten-regression/emsdk/upstream/emscripten/cache/sysroot/lib/wasm32-emscripten/libjpeg.a(jdinput.c.o): relocation R_WASM_TABLE_INDEX_SLEB cannot be used against symbol `consume_markers`; recompile with -fPIC
```

Author: ryanking13

test/test_other.py

@@ -2559,6 +2559,42 @@ def test_libjpeg(self):
                  cflags=['--embed-file', 'screenshot.jpg', '--use-port=libjpeg'],
                  args=['screenshot.jpg'])
 
+  @requires_network
+  def test_side_module_with_ports(self):
+    # Verify that ports can be used in side modules, and that the resulting
+    # side module can be used from a main module.
+    create_file('side.c', r'''
+      #include <stdio.h>
+      #include <gif_lib.h>
+
+      void gif_side_test(void) {
+        ColorMapObject* map = GifMakeMapObject(2, NULL);
+        if (map) {
+          GifFreeMapObject(map);
+        }
+        puts("gif side ok");
+      }
+    ''')
+
+    create_file('main.c', r'''
+      #include <stdio.h>
+
+      void gif_side_test(void);
+
+      int main() {
+        gif_side_test();
+        puts("main ok");
+        return 0;
+      }
+    ''')
+
+    self.emcc('side.c', args=['-sSIDE_MODULE', '-sUSE_GIFLIB', '-lgif', '-o', 'libgif_side.so'])
+    self.assertExists('libgif_side.so')
+
+    # Linking the side module into the main module should cause it to be loaded
+    # automatically at runtime.
+    self.do_runf('main.c', 'gif side ok\n', cflags=['-sMAIN_MODULE=2', 'libgif_side.so'])
+
   @requires_network
   @also_with_wasm64
   def test_bullet(self):

tools/cache.py

@@ -124,7 +124,7 @@ def get_lib_dir(absolute):
       subdir.append('thinlto')
     else:
       subdir.append('lto')
-  if settings.MAIN_MODULE:
+  if settings.MAIN_MODULE or settings.SIDE_MODULE:
     subdir.append('pic')
   if subdir:
     path = Path(path, '-'.join(subdir))

tools/system_libs.py

@@ -61,7 +61,7 @@ def get_base_cflags(build_dir, force_object_files=False, preprocess=True):
   flags = ['-g', '-sSTRICT', '-Werror']
   if settings.LTO and not force_object_files:
     flags += ['-flto=' + settings.LTO]
-  if settings.MAIN_MODULE:
+  if settings.MAIN_MODULE or settings.SIDE_MODULE:
     # Explicitly include `-sMAIN_MODULE` when building system libraries.
     # `-fPIC` alone is not enough to configure trigger the building and
     # caching of `pic` libraries (see `get_lib_dir` in `cache.py`)