build with -march=native by default

nim.cfg

@@ -21,6 +21,13 @@
   -d:"chronicles_colors=off"
 @end
 
+# This helps especially for 32-bit x86, which sans SSE2 and newer instructions
+# requires quite roundabout code generation for cryptography, and other 64-bit
+# and larger arithmetic use cases, along with register starvation issues. When
+# engineering a more portable binary release, this should be tweaked but still
+# use at least -msse2 or -msse3.
+--passC:"-march=native"
+
 --threads:on
 --opt:speed
 --excessiveStackTrace:on