Add -mstrict-align to the gcc options

Add -mstrict-align to the gcc options

ARMv8 architecture allows unaligned memory accesses. However,
Trusted Firmware disables such feature by setting the SCTLR_A_BIT
and SCTLR_SA_BIT in the SCTLR_EL3 register (it enables alignment
checks).

This patch adds -mstrict-align to the gcc build options. Although
there are not explicit unaligned memory accesses in Trusted Firmware,
this flag will tell the compiler not to use them.

Fixes ARM-software/tf-issues#294

Change-Id: I69748c6cf28504be9ca3dc975a331d14459c9ef1