[WebAssembly] Disable custom NaN payload tests

Summary:
These tests fail on 32-bit builds because NaN payload bits in floating point
immediates are not necessarily preserved through compilation. This is because
the MC layer uses native doubles to store these values. The tests will be
reenabled once this problem has been fixed or deleted if we decide we don't care
about lowering payload bits.

Reviewers: aheejin, dschuff

Subscribers: sbc100, jgravelle-google, sunfish, llvm-commits

Differential Revision: https://reviews.llvm.org/D54353

llvm-svn: 346558