s390x: allow clang to emit fused multiply-adds (replicates gcc's default behavior)

s390x: allow clang to emit fused multiply-adds (replicates gcc's default behavior)

gcc's default setting for floating-point expression contraction is
"fast", which allows the compiler to emit fused multiply adds instead of
separate multiplies and adds (amongst others). Fused multiply-adds,
which assembly kernels typically apply, also bring a significant
performance advantage to the C implementation for matrix-matrix
multiplication on s390x. To enable that performance advantage for builds
with clang, add -ffp-contract=fast to the compiler options.

Signed-off-by: Marius Hillenbrand <mhillen@linux.ibm.com>