[flang] Add -fppc-native-vector-element-order option to control the element...

[flang] Add -fppc-native-vector-element-order option to control the element order in PowerPC vector types

This patch also adds a LIT test for the vec_cvf intrinsic that
can be affected by the option.

Co-authored-by: Mark Danial <Mark.Danial@ibm.com>
Co-authored-by: Daniel Chen <cdchen@ca.ibm.com>

Differential Revision: https://reviews.llvm.org/D155852