buildsys: minor fixes in Makefile.arch for microblaze

Do not include Makefile.commonarch directly from within arch specific
Makefile, as it is already done in parent Maefile.in.

Signed-off-by: Carmelo Amoroso <carmelo.amoroso@st.com>

libc/sysdeps/linux/microblaze/Makefile.arch

@@ -10,5 +10,3 @@ CSRC := mmap.c clone.c fixdfsi.c
 SSRC := setjmp.S __longjmp.S vfork.S
 
 ARCH_HEADERS := floatlib.h
-
-include $(top_srcdir)libc/sysdeps/linux/Makefile.commonarch