Compile _decimal without asserts and update benchmark results.

Doc/whatsnew/3.3.rst

@@ -1119,7 +1119,7 @@ at http://www.bytereef.org/mpdecimal/quickstart.html.
    +---------+-------------+--------------+-------------+
    |         |  decimal.py |   _decimal   |   speedup   |
    +=========+=============+==============+=============+
-   |   pi    |    38.89s   |    0.38s     |    100x     |
+   |   pi    |    42.02    |    0.345     |    120x     |
    +---------+-------------+--------------+-------------+
    | telco   |   172.19s   |    5.68s     |     30x     |
    +---------+-------------+--------------+-------------+

setup.py

@@ -1897,7 +1897,7 @@ class PyBuildExt(build_ext):
 
     def _decimal_ext(self):
         extra_compile_args = []
-        undef_macros = ['NDEBUG']
+        undef_macros = []
         if '--with-system-libmpdec' in sysconfig.get_config_var("CONFIG_ARGS"):
             include_dirs = []
             libraries = ['mpdec']