bsd.compat.mk: Provide new CPP and sub-make variables

Currently the only way to detect for a libcompat build is to consult
whether COMPAT_32BIT is defined (or equivalent, for downstreams with
other libcompats or past releases with libsoft as COMPAT_SOFTFP). There
are two issues with this:

1. COMPAT_32BIT is a new naming scheme that doesn't match the libcompat
   name, which is unnecessary deviation.
2. When multiple libcompats exist, everywhere that needs to detect a
   libcompat must check each variable in turn, despite the fact that it
   normally just wants to know if this is a libcompat build and perhaps
   what ${LIBCOMPAT} and/or ${libcompat} are for it.

As a result, far too many places in the tree need to know about the set
of possible libcompats.

Instead, introduce two new CPP and sub-make variables, COMPAT_LIBCOMPAT
and COMPAT_libcompat, which give the values for ${LIBCOMPAT} and
${libcompat} respectively, so that uses can be made parameterised. For
when code really does need to know the specific libcompat, Makefiles can
perform a string comparison, but the C preprocessor cannot, so introduce
an additional CPP-only COMPAT_LIB${LIBCOMPAT} which is intended to
replace the inconsistently-named COMPAT_32BIT (which will be removed in
future). Uses of this new variable should still be kept to a minimum,
however, given the code duplication needed for new libcompats.

Reviewed by:	emaste, brooks, jhb
Differential Revision:	https://reviews.freebsd.org/D40922

share/mk/bsd.compat.mk

@@ -115,11 +115,17 @@ WORLDTMP?=		${SYSROOT}
 LIB${_LIBCOMPAT}_OBJTOP?=	${OBJTOP}/obj-lib${_libcompat}
 
 LIB${_LIBCOMPAT}CFLAGS+=	${LIB${_LIBCOMPAT}CPUFLAGS} \
+				-DCOMPAT_LIBCOMPAT=\"${_LIBCOMPAT}\" \
+				-DCOMPAT_libcompat=\"${_libcompat}\" \
+				-DCOMPAT_LIB${_LIBCOMPAT} \
 				--sysroot=${WORLDTMP} \
 				${BFLAGS}
 
 LIB${_LIBCOMPAT}LDFLAGS+=	-L${WORLDTMP}/usr/lib${_libcompat}
 
+LIB${_LIBCOMPAT}WMAKEFLAGS+=	COMPAT_LIBCOMPAT=${_LIBCOMPAT} \
+				COMPAT_libcompat=${_libcompat}
+
 LIB${_LIBCOMPAT}WMAKEENV+=	MACHINE=${LIB${_LIBCOMPAT}_MACHINE}
 LIB${_LIBCOMPAT}WMAKEENV+=	MACHINE_ARCH=${LIB${_LIBCOMPAT}_MACHINE_ARCH}