Add s390(x) port

[sharkcz]

This is a s390(x) port with the following results

test/test-double
testing double (without inline functions)

Test suite completed:
  1118 test cases plus 932 tests for exception flags executed.
  All tests passed successfully.
test/test-float
testing float (without inline functions)
Failure: Test: yn (10, 2.0) == -129184.54220803928264
Result:
 is:         -1.29184562500000000000e+05  -0x1.f8a09000000000000000p+16
 should be:  -1.29184539062500000000e+05  -0x1.f8a08a00000000000000p+16
 difference:  2.34375000000000000000e-02   0x1.80000000000000000000p-6
 ulp       :  3.0000
 max.ulp   :  1.0000

Test suite completed:
  1101 test cases plus 923 tests for exception flags executed.
  1 errors occurred.

[ViralBShah]

These changes look ok. Thanks.

[sharkcz]

Is there anything we can do for the precision issue with yn (10, 2.0) ?

[ViralBShah]

Perhaps open a separate issue to track it? I say it is good enough for now.