build: turn on debug-safe optimizations with -Og

The resulting binary is still easy to inspect in gdb but is not as dog
slow.  The 'parallel' test suite, although it takes several minutes to
complete, now finishes without any tests timing out.

common.gypi

@@ -14,7 +14,7 @@
     'node_tag%': '',
     'uv_library%': 'static_library',
 
-    # Default to -O0 for debug builds.
+    # Default to -Og for debug builds.
     'v8_optimized_debug%': 0,
 
     # Enable disassembler for `--print-code` v8 options
@@ -54,7 +54,7 @@
           'v8_enable_handle_zapping%': 1,
         },
         'defines': [ 'DEBUG', '_DEBUG' ],
-        'cflags': [ '-g', '-O0' ],
+        'cflags': [ '-g', '-Og' ],
         'conditions': [
           ['target_arch=="x64"', {
             'msvs_configuration_platform': 'x64',