x86: merge IAMCU and SYS V core arch code

Having two parallel implementations is a maintenance issue, especially
when some strategically placed #ifdefs will suffice.

We prefer the ASM versions for SYS V, as we need complete control of
the emitted assembly for interrupt handling and context switching.
The SYS V code is far more mature. IAMCU C code has known issues with
-fomit-frame-pointer.

The only difference between the two calling conventions is that the
first three function arguments are provided in eax, edx, ecx instead
of on the stack.

Issue: ZEP-49
Change-Id: I9245e4b0ffbeb6d890a4f08bc8a3a49faa6d8e7b
Signed-off-by: Andrew Boie <andrew.p.boie@intel.com>