core: simplify pm_call usage

core: simplify pm_call usage

Simplify the pm_call usage:

- pm_call now saves and restores all registers
  callees that want to return values in registers can mess with
  the frame on the stack
- the PM function now enters with EAX = EBP = RM stack
  (EAX for the use of C code, EBP for assembly)
- the RM stack frame matches com32sys_t
- pmcall.inc defined macros for the stack frame based on EBP

Signed-off-by: H. Peter Anvin <hpa@zytor.com>