cpus: fix wrong define name

While the configure script generates TARGET_SUPPORTS_MTTCG define, one
of the define is cpus.c is checking wrong name: TARGET_SUPPORT_MTTCG

Signed-off-by: Nikunj A Dadhania <nikunj@linux.vnet.ibm.com>
Signed-off-by: Alex Bennée <alex.bennee@linaro.org>
