multimedia/x265: Fix build dependencies and some minor clean-ups

[diizzy]

Summary

Only require NASM on i386 and amd64
No need to pass flags using both CFLAGS and CPPFLAGS
Strip -O2 when enabling OPTIMIZED_CFLAGS to avoid setting multiple -O FLAGS

Test Plan

Compile and Run-time tested on:
aarch64, Orange Pi PC2, 13.0-CURRENT
armv7, Orange Pi PC, 13.0-CURRENT
amd64, Dell T20 (Intel i7-3770), 13.0-CURRENT

[diizzy]

I've mailed maintainer separately and linked to this differential

[diizzy]

Migrated to https://bugs.freebsd.org/bugzilla/show_bug.cgi?id=239510