OpenCL: Defining __ENDIAN_LITTLE__ and fix target endianness

OpenCL requires __ENDIAN_LITTLE__ be set for little endian targets.
The default for targets was also apparently big endian, so AMDGPU
was incorrectly reported as big endian. Set this from the triple
so targets don't have another place to set the endianness.

git-svn-id: https://llvm.org/svn/llvm-project/cfe/trunk@280787 91177308-0d34-0410-b5e6-96231b3b80d8