S/390: Implement vector copysign.

S/390: Implement vector copysign.

This patch implements the vector copysign operation using vector select and a
signbit mask.

gcc/ChangeLog:

2019-02-08  Robin Dapp  <rdapp@linux.ibm.com>

	* config/s390/vector.md: Implement vector copysign.

gcc/testsuite/ChangeLog:

2019-02-08  Robin Dapp  <rdapp@linux.ibm.com>

	* gcc.target/s390/vector/vec-copysign-execute.c: New test.
	* gcc.target/s390/vector/vec-copysign.c: New test.

git-svn-id: svn+ssh://gcc.gnu.org/svn/gcc/trunk@268697 138bc75d-0d04-0410-961f-82ee72b054a4