Switch Linux futex emulation to native ucas_int(), remove unecessary
futex-specific assembler routines.

Fixes PR/40490. Reviewed by <ad>.

catch up with i386 changes.

merge ktrace-lwp.

First work on COMPAT_LINUX/amd64
Process startup and dynamiclinking work, but processes hang due to
Linux arch_prctl(2) not being really supported yet.