s390x: enable S/DGEMM block with explicit loop unrolling + interleaving with clang

s390x: enable S/DGEMM block with explicit loop unrolling + interleaving with clang

The code for SGEMM 16x4 and DGEMM 8x4 blocks on z14 and z15 uses
explicit unrolling and interleaving to improve performance. The code
employs an empty inline asm statement with operands that constrain the
compiler's instruction scheduling and thereby enforce proper overlapping
of load and compute phases. Fix an ifdef to apply that for clang builds,
as well.

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