libsysdecode: Migrate from COMPAT_32BIT to generic COMPAT_LIBCOMPAT

The use isn't any more generic, just the variable itself, which will
allow COMPAT_32BIT to be removed. The fact we even have to check
COMPAT_LIBCOMPAT here in order to pass the right flags to CPP points at
our libcompat infrastructure not suitably modifying the CPP variable
(which we barely use for world; this and bsd.symver.mk are the two
uses, and the latter could benefit from the right flags too), but this
change doesn't attempt to fix that.

See commit 8fad2cda93c7 ("bsd.compat.mk: Provide new CPP and sub-make
variables") for the context behind this change.

Reviewed by: emaste, imp, brooks, jhb
Differential Revision: https://reviews.freebsd.org/D40930