toolchain/gcc: the ARC version of gcc needs host-flex/host-bison

For some reason (probably because the ARC changes modify some lex/yacc
files without updating their pre-generated variants, or because the
date/time of the pre-generated files is not correct), building the ARC
gcc requires host-flex and host-bison.

We have tested 4.2 for AVR, 4.3 and 4.4 for ARM, and none of those
need host-flex or host-bison to be installed, so only the 4.4 for ARC
seems to be affected.

Fixes the build failure visible at
http://autobuild.buildroot.org/results/673c6262e3dde8ee8dd28204d814097e6ba8f8e9/build-end.log

.

Signed-off-by: Thomas Petazzoni <thomas.petazzoni@free-electrons.com>
Signed-off-by: Peter Korsgaard <jacmet@sunsite.dk>