bsd.compat.mk: Allow finding non-internal libraries

Currently only libexec/rtld-elf32 uses internal LIBC_NOSSP_PIC during
the build but it gets it directly from the objdir rather than a sysroot.
For example, /usr/obj/usr/src/amd64.amd64/obj-lib32/lib/libc/libc_nossp_pic.a.
We don't stage lib32 libraries in WORLDTMP/usr/lib32 and doing so doesn't
buy much.  If we want to use a staged lib32 library then we need to look in
LIBCOMPATTMP where they were staged.  For example if LIBC_PIC were wanted then
look for /usr/obj/usr/src/amd64.amd64/obj-lib32/tmp/usr/lib32/libc_pic.a.

Reported by:	rlibby
Reviewed by:	rlibby
Sponsored by:	Dell EMC
Differential Revision:	https://reviews.freebsd.org/D27648

share/mk/bsd.compat.mk

@@ -161,6 +161,7 @@ LIBCOMPATCFLAGS+=	-B${LIBCOMPATTMP}/usr/lib${libcompat}
 .if defined(WANT_COMPAT)
 LIBDIR_BASE:=	/usr/lib${libcompat}
 _LIB_OBJTOP=	${LIBCOMPAT_OBJTOP}
+LIBDESTDIR:=	${LIBCOMPATTMP}
 CFLAGS+=	${LIBCOMPATCFLAGS}
 LDFLAGS+=	${CFLAGS} ${LIBCOMPATLDFLAGS}
 MACHINE=	${LIBCOMPAT_MACHINE}

share/mk/bsd.libnames.mk

@@ -8,7 +8,7 @@
 .error bsd.libnames.mk cannot be included directly.
 .endif
 
-LIBDESTDIR=	${SYSROOT:U${DESTDIR}}
+LIBDESTDIR?=	${SYSROOT:U${DESTDIR}}
 
 .sinclude <src.libnames.mk>