Fix vec_copysign implementations per issue #158, Part 1B.

[munroesj52]

Rebased after the add/subqpo merge. Cliose #158
Seems the GCC (and Clang followed) initially reversed the operands
for vec_cpsgn() (so the sign is copied from operand b into a).
This differs from the Intrinsic reference and ISA that say the sign
is copied from operand a into b.

Unfortunately PVECLIB implementations of vec_copysignf32(),
vec_copysignf64(), and vec_copysignf128() duplicated the results of
original GCC vec_cpsgn() for the implementation,

This has been reported as a bug and now GCC and Clang are in the
process of "fixing" this bug to match the Intrinsic reference guide.
The fix will be applied to currently supported versions but older
compiler version will remain unchanged.

This change set will correct the PVECLIB copysign implementation
to match the intrinsic reference manual. We will the macro
PVECLIB_CPSGN_FIXED to isolate the PVECLIB implementations from
the compiler changes.

* src/pveclib/vec_f128_ppc.h (vec_copysignf128): Change to
match operand order from Vector Intrinsic Reference.
(vec_xscvsqqp [_ARCH_PWR9]): Use correct vec_copysignf128
operand order.
* src/pveclib/vec_f32_ppc.h (vec_copysignf32): Change to match
operand order from Vector Intrinsic Reference.
* src/pveclib/vec_f64_ppc.h (vec_copysignf64): Change to match
operand order from Vector Intrinsic Reference.

* src/testsuite/arith128_test_f128.c (test_copysignf128):
Define new __float128 const f128_nsnan.
Reverse test operands and results to match corrected
vec_copysignf128() implementation. Remove duplicated tests.
* src/testsuite/arith128_test_f32.c (test_float_cpsgn):
Reverse test operands and results to match corrected
vec_copysignf32() implementation.
* src/testsuite/arith128_test_f64.c (test_double_cpsgn):
Reverse test operands and results to match corrected
vec_copysignf64() implementation.

* src/testsuite/vec_f32_dummy.c (test_vec_copysignf32):
new compile test.
* src/testsuite/vec_f64_dummy.c (test_vec_copysignf64):
new compile test.

Signed-off-by: Steven Munroe munroesj52@gmail.com

[munroesj52]

We need to think about cherry picking this change for 1.0.4-4 -> 1.0.4-5.

But this is an API change, so should this be a version bump?

[munroesj52]

This has aged long enough. So merging so I can move on.