audio/qmidiarp: fix build on non-x86
The options -msse, -msse2, and -mfpmath=sse only apply to x86 targets.
They are the default on amd64, so only set them on i386.
Approved by: portmgr (build fix blanket)
MFH: 2026Q1
(cherry picked from commit 21a28646fdd6de37f9a9e2429ca93fce350cb1cb)