[ARM][XO] Execute-only miscompiles double literals for big-endian

Summary:
With -mbig-endian -mexecute-only and targeting an fpu,
an incorrect sequence of movw/movt was generated to construct a double literal.
The test suite was hardwired to check these wrong values.

The fault was caused by the explicit word swap in LowerConstantFP().

With -mbig-endian -mexecute-only -mfpu=none, a correct sequence of
movw/movt is generated to construct a double literal.
The test suite did not test this no fpu case.

The test suite expected values have been corrected.
The test file is updated to add testing of fpu=none case

Reviewers: christof, llvm-commits, dmgreen

Reviewed By: dmgreen

Subscribers: dmgreen, kristof.beyls, hiraditya, danielkiss

Tags: #llvm

Differential Revision: https://reviews.llvm.org/D81259

Change-Id: Ia3737df243218c89c82f02b7f9f4032ecd5a3917