build: use LC_ALL of C for maximum compatibility

LC_ALL=en_US breaks on some systems (notably the SmartOS 16
configuration in our CI). Use LC_ALL=C instead.

configure

@@ -672,10 +672,7 @@ def get_xcode_version(cc):
 def get_gas_version(cc):
   try:
     custom_env = os.environ.copy()
-    # smartos (a.k.a. sunos5) does not have the en_US locale, and will give:
-    # `setlocale: LC_ALL: cannot change locale (en_US): Invalid argument`
-    if 'sunos' not in sys.platform:
-      custom_env["LC_ALL"] = "en_US"
+    custom_env["LC_ALL"] = "C"
     proc = subprocess.Popen(shlex.split(cc) + ['-Wa,-v', '-c', '-o',
                                                '/dev/null', '-x',
                                                'assembler',  '/dev/null'],