package/gcc: disable libitm for sparc <v9

libitm (transactional memory) needs SPARC V9+ ISA, otherwise when
enabling C++ the toolchain fails to build:

/tmp/cclQ6hrD.s: Assembler messages:
/tmp/cclQ6hrD.s:1261: Error: Architecture mismatch on "rd".
/tmp/cclQ6hrD.s:1261:  (Requires v9|v9a|v9b; requested architecture is
v8.)
Makefile:517: recipe for target 'beginend.lo' failed
make[5]: *** [beginend.lo] Error 1

So disable it for our current (v8, leon3) support.

Signed-off-by: Gustavo Zacarias <gustavo@zacarias.com.ar>
Signed-off-by: Thomas Petazzoni <thomas.petazzoni@free-electrons.com>