tools: always include llvm_version in config

Also used in common.gypi to check whether a flag is needed or not
based on llvm version.

PR-URL: nodejs#14077
Reviewed-By: Ben Noordhuis <info@bnoordhuis.nl>
Reviewed-By: Refael Ackermann <refack@gmail.com>

Author: nanaya

configure

@@ -640,12 +640,14 @@ def check_compiler(o):
     # to a version that is not completely ancient.
     warn('C compiler too old, need gcc 4.2 or clang 3.2 (CC=%s)' % CC)
 
-  # Need llvm_version or gas_version when openssl asm files are compiled
+  if is_clang:
+    o['variables']['llvm_version'] = get_llvm_version(CC)
+
+  # Need xcode_version or gas_version when openssl asm files are compiled.
   if options.without_ssl or options.openssl_no_asm or options.shared_openssl:
     return
 
   if is_clang:
-    o['variables']['llvm_version'] = get_llvm_version(CC)
     if sys.platform == 'darwin':
       o['variables']['xcode_version'] = get_xcode_version(CC)
   else: