graphics/png: exclude SIMD from armv{6,7} in QEMU_EMULATING

The current version added an assert for ARM NEON detection, which
is not supported under QEMU_EMULATING, so the build fails. Prevent
exposing the SIMD option in this case.

Approved by: tcberner (mentor)
Differential Revision: https://reviews.freebsd.org/D37212