docs/manual: update the list of libffi unsupported archs

libffi supports ARC since commit 34c2afeb750 ("libffi: back-port support
for ARC") in 2014.

Add nds32 and ARMv7-M to the list.

Cc: Thomas De Schampheleire <thomas.de_schampheleire@nokia.com>
Signed-off-by: Baruch Siach <baruch@tkos.co.il>
Signed-off-by: Thomas Petazzoni <thomas.petazzoni@bootlin.com>

docs/manual/known-issues.txt

@@ -7,7 +7,7 @@
   if such options contain a +$+ sign. For example, the following is known
   to break: +BR2_TARGET_LDFLAGS="-Wl,-rpath=\'$ORIGIN/../lib'"+
 
-* The +libffi+ package is not supported on the SuperH 2 and ARC
+* The +libffi+ package is not supported on the SuperH 2, nds32, and ARMv7-M
   architectures.
 
 * The +prboom+ package triggers a compiler failure with the SuperH 4