Special function names

[Robert Swindells <rjs%fdy2.co.uk@localhost>]

Joerg pointed out that gcc has a list of function names that it special
cases, this was the cause of x86/vm_machdep.c failing to compile:

<http://mail-index.netbsd.org/current-users/2019/10/17/msg036799.html>

Any thoughts on whether we should remove some or all of the names
from the list in our build of gcc ?

The list is: "setjmp", "sigsetjmp", "savectx", "vfork", "getcontext"

There are similar declarations in clang.