Restrict AES-NI support to ELF platforms for now

Since we explicitly specify the ELF object format when building
iaesx86.s or iaesx64.s, we need to restrict it to operating systems we
know to be ELF platforms.  Otherwise we can break the build on OS X,
which uses the Mach-O object format.

ticket: 7812
target_version: 1.12.1
tags: pullup