Build debug kernels with -O2.

LLVM 11 changed the meaning of '-O' from '-O2' to '-O1', which resulted
in debug kernels (with 'makeoptions DEBUG=-g') being built with inlining
disabled, causing severe performance hit.

The -O2 was already being used for building amd64, powerpc, and powerpcspe.

Discussed with: jrtc27, arichardson, bdragon, jhibbits
Sponsored by: DARPA
Differential Revision: https://reviews.freebsd.org/D26471