clang: 32 bit compilation should include march32-directive
clang: 32 bit compilation should include march32-directive

This is done for other compilers, and without this some code does not
compile, like inline assembly code.

Fixes ARM-software/tf-issues#657.

Signed-off-by: Yann Gautier <e753f6969c288905606e3c07464886d4f1fc1be0@st.com>

achingupta/arm-trusted-firmware,achingupta/arm-trusted-firmware