MFH: r535336

security/nss: back out my own changes for troubleshooting, disable VSX when VSX option is off

I had pre-build commented out. This is needed to actually fix build.
NSS_DISABLE_ALTIVEC doesn't actually disable AltiVec.
It only disables VSX, because NSS doesn't support AltiVec.

Approved by: portmgr (fix build blanket)