Ensure system libraries are built on demand (#19405)

Previously, when explicitly linking against a system library (e.g.
`-lc`) that had not been built earlier, wasm-ld would raise an error
indicating that the library could not be found.

Use the `get_link_flag()` function to ensure that it triggers a
build if the system library is not present in the cache.

emcc.py

@@ -4184,8 +4184,8 @@ def process_libraries(state, linker_inputs):
     # let wasm-ld handle that.  However, we do want to map to the correct variant.
     # For example we map `-lc` to `-lc-mt` if we are building with threading support.
     if 'lib' + lib in system_libs_map:
-      lib = system_libs_map['lib' + lib]
-      new_flags.append((i, '-l' + strip_prefix(lib.get_base_name(), 'lib')))
+      lib = system_libs_map['lib' + lib].get_link_flag()
+      new_flags.append((i, lib))
       continue
 
     if building.map_and_apply_to_settings(lib):

test/test_other.py

@@ -12110,8 +12110,6 @@ def test_offset_convertor_plus_wasm2js(self):
 
   def test_standard_library_mapping(self):
     # Test the `-l` flags on the command line get mapped the correct libraries variant
-    self.run_process([EMBUILDER, 'build', 'libc-mt-debug', 'libcompiler_rt-mt', 'libdlmalloc-mt'])
-
     libs = ['-lc', '-lbulkmemory', '-lcompiler_rt', '-lmalloc']
     err = self.run_process([EMCC, test_file('hello_world.c'), '-pthread', '-nodefaultlibs', '-v'] + libs, stderr=PIPE).stderr
 

tools/system_libs.py

@@ -426,7 +426,7 @@ def get_link_flag(self):
     This will trigger a build if this library is not in the cache.
     """
     fullpath = self.build()
-    # For non-libaries (e.g. crt1.o) we pass the entire path to the linker
+    # For non-libraries (e.g. crt1.o) we pass the entire path to the linker
     if self.get_ext() != '.a':
       return fullpath
     # For libraries (.a) files, we pass the abbreviated `-l` form.