Do not attempt to build libdisk, libthr and libc_r for arm.

lib/Makefile

@@ -26,7 +26,7 @@ SUBDIR=	${_csu} libcom_err libcrypt libkvm msun libmd libncurses \
 	libnetgraph libradius librpcsvc libsbuf libtacplus libutil \
 	${_libypclnt} ${_compat} libalias libarchive ${_libatm} ${_libbind} \
 	${_libbluetooth} libbsnmp libbz2 libc ${_libc_r} \
-	libcalendar libcam libcompat libdevinfo libdevstat libdisk \
+	libcalendar libcam libcompat libdevinfo libdevstat ${_libdisk} \
 	libedit libexpat libfetch libform libftpio libgeom ${_libio} libipsec \
 	libipx libisc libkiconv libmenu ${_libmilter} ${_libmp} ${_libncp} \
 	${_libngatm} libopie libpam libpanel libpcap \
@@ -43,7 +43,8 @@ _csu=csu
 .endif
 
 # libc_r is obsolete on ia64.
-.if ${MACHINE_ARCH} != "ia64" && ${MACHINE_ARCH} != "powerpc"
+.if ${MACHINE_ARCH} != "ia64" && ${MACHINE_ARCH} != "powerpc" && \
+	${MACHINE_ARCH} != "arm"
 .if !defined(NOLIBC_R)
 _libc_r=	libc_r
 .endif
@@ -96,12 +97,16 @@ _libpthread=	libpthread
 .endif
 .endif
 
-.if ${MACHINE_ARCH} != "amd64"
+.if ${MACHINE_ARCH} != "amd64" && ${MACHINE_ARCH} != "arm"
 .if !defined(NOLIBTHR)
 _libthr=	libthr
 .endif
 .endif
 
+.if ${MACHINE_ARCH} != "arm"
+_libdisk=	libdisk
+.endif
+
 .if !defined(NOCRYPT) && !defined(NO_OPENSSL)
 _libmp=		libmp
 .endif