Introduce NCG config flag and add helper

... for testing presence of NCG

This commit adds a criterion for checking whether we can expect sensible
output from --ddump-asm.

testsuite/config/ghc

@@ -158,6 +158,8 @@ def get_compiler_info():
     s = re.sub('[\r\n]', '', s)
     rtsInfoDict = dict(eval(s))
 
+    config.have_ncg = compilerInfoDict.get("Have native code generator", "NO") == "YES"
+
     # external interpreter needs RTS linker support
     # If the field is not present (GHC 8.0 and earlier), assume we don't
     # have -fexternal-interpreter (though GHC 8.0 actually does)

testsuite/driver/testlib.py

@@ -440,6 +440,9 @@ def cygwin( ):
 def have_vanilla( ):
     return config.have_vanilla
 
+def have_ncg( ):
+    return config.have_ncg
+
 def have_dynamic( ):
     return config.have_dynamic
 
@@ -1174,7 +1177,7 @@ def check_stats(name, way, stats_file, range_fields):
                 metric_result = failBecause('no such stats metric')
             else:
                 actual_val = int(field_match.group(1))
-                
+
                 # Store the metric so it can later be stored in a git note.
                 perf_stat = metric_dict(name, way, metric, actual_val)
                 change = None