GCC options on EC2

AWS second-generation instances (such as my favorite, m3.medium) claim to launch with Xeon E5-2670 v2 processors, but sometimes the v1 variant.  These chips are Ivy Bridge and Sandy Bridge, respectively.  AFAICT, this means that AMIs with code compiled for Sandy Bridge should run on any second-generation instance, but may not necessarily run on previous-generation instances.  What little I can find online about previous-generation instances shows a mix of 90nm Opteron parts (having only up to SSE3) and Intel parts from Penryn-based Harpertown up through Westmere.

GCC changed their -march options in the recently-released version 4.9.  GCC 4.8, which notably shipped in Ubuntu 14.04 (Trusty Tahr), used some inscrutable aliases; the equivalent to 4.9's sandybridge is corei7-avx for instance.  Here's a table of the subset that's most interesting to me:

GCC 4.8	GCC 4.9
corei7	nehalem
–	westmere
corei7-avx	sandybridge
corei7-avx-i	ivybridge
corei7-avx2	haswell

Broadwell and Westmere are not explicitly supported in the older release.  Based on its definition in the gcc-4.9 sources, I believe the equivalent set of flags for Westmere in gcc 4.8 would be -march=corei7 -maes -mpclmul.  And naturally, the corei7-avx2 option for Haswell would be the best for targeting Broadwell.