makesyscalls: Add a way to include per-ABI headers

When the string %%ABI_HEADERS%% is found in syscalls.master, replace
it with the contents of the abi_headers config variable. This allows
an ABI-specific syscalls.conf to add lines like:

#include <compat/freebsd32/freebsd32.h>

when working from a shared syscalls.master.

Reviewed by: kevans