makesyscalls: add override of ABI change detection

While we can detect most ABI changes through analysis of
syscalls.master with suitable annotations, to cases are handled
in the core implementation and others have changes that can not be
infered. Add two new config variables syscall_abi_change and
syscall_no_abi_change which override the detected value. Both are
space-seperated lists of syscall names.

Reviewed by: kevans