trace: Make trace point code patching thread/smp safe

Also, move platform dependent fast dispatch to platform arch code tree(s)

The patching code is a bit more complex than would seem immediately (or
even factually) neccesary. However, depending on cpu, there might be
issues with trying to code patch across cache lines (unaligned).
To be safe, we do it with the old 16-bit jmp + write + finish dance.

[avi: fix up build.mk]

Signed-off-by: Calle Wilund <calle@cloudius-systems.com>
Signed-off-by: Avi Kivity <avi@cloudius-systems.com>