Makefile.inc1: add LIBCOMPAT_INCLUDE_DIRS to reduce arch ifdefs

In preparation for adding support for building lib32 on arm64,
add a list of architecture-specific include directories,
LIBCOMPAT_INCLUDE_DIRS, then replace the architecture-specific
ifdefs throughout the file with simple loops. Another commit
will add a definition of LIBCOMPAT_INCLUDE_DIRS for aarch64.

Reviewed by: jrtc27
Differential Revision: https://reviews.freebsd.org/D40977