[PATCH] Add optimized addmul_1 and submul_1 for IBM z13

[Marius Hillenbrand]

Hi,

These patches add IBM z13 as a new s390_64 CPU level to mpn and add optimized
versions of addmul_1 and submul_1 that exploit the SIMD extensions that were
introduced with the IBM z13 generation. Both implementations share the same
structure and use 128-bit add/subtract ops in vector registers with carry/borrow
bits in registers.

Tested with the regression test suite and stressed with tests/devel/anymul_1.c

Please review and merge.

I got started with addmul_1 since it felt challenging and instructive enough,
yet will look into the other ops, as well.

Thanks,
Marius