Build crt1.o as a wasm object file (emscripten-core#9883)

The underlying issue is that if crt1.o is built with
WASM_OBJECT_FILES=0 then it's a bitcode file. That appears
to be linked together with the user's main before the linker
would create the __original_main function. As a result, the trick
with making that weak doesn't work.

The proposed solution here is to build crt1.o as a wasm object
file in all cases. Then it's linked in at the very end, and the
optimization works.

Author: kripken

tests/other/metadce/mem_no_argv_O3_STANDALONE_WASM_flto.exports

@@ -0,0 +1,3 @@
+_start
+main
+memory

tests/other/metadce/mem_no_argv_O3_STANDALONE_WASM_flto.funcs

@@ -0,0 +1,5 @@
+$__original_main
+$_start
+$dlmalloc
+$main
+$sbrk

tests/other/metadce/mem_no_argv_O3_STANDALONE_WASM_flto.imports

@@ -0,0 +1 @@
+proc_exit

tests/other/metadce/mem_no_argv_O3_STANDALONE_WASM_flto.sent

@@ -0,0 +1 @@
+proc_exit

tests/test_other.py

@@ -8250,6 +8250,10 @@ def test_metadce_hello_fastcomp(self, *args):
     # Growth support code is in the wasm
     'O3_grow_standalone': ('mem.c', ['-O3', '-s', 'ALLOW_MEMORY_GROWTH', '-s', 'STANDALONE_WASM'],
                            4, [], [], 6449,  4,  3,  6),         # noqa
+    # without argc/argv, no support code for them is emitted, even with lto
+    'O3_standalone_narg_flto':
+                          ('mem_no_argv.c', ['-O3', '-s', 'STANDALONE_WASM', '-flto'],
+                           1, [], [], 6309,  1,  3,  5),         # noqa
   })
   @no_fastcomp()
   def test_metadce_mem(self, filename, *args):

tools/system_libs.py

@@ -766,12 +766,17 @@ class crt1(MuslInternalLibrary):
   src_dir = ['system', 'lib', 'libc']
   src_files = ['crt1.c']
 
+  force_object_files = True
+
   def get_ext(self):
     return '.o'
 
   def can_use(self):
     return shared.Settings.STANDALONE_WASM
 
+  def can_build(self):
+    return shared.Settings.WASM_BACKEND
+
 
 class libc_extras(MuslInternalLibrary):
   """This library is separate from libc itself for fastcomp only so that the