[WebAssembly] Mark immediates.ll as XFAILed on MIPS hosts

Usually MIPS hosts uses a legacy (non IEEE 754-2008) encoding for NaNs.
Tests like `nan_f32` failed in attempt to compare hard-coded IEEE
754-2008 NaN value and a legacy NaN value provided by a system.

git-svn-id: https://llvm.org/svn/llvm-project/llvm/trunk@346773 91177308-0d34-0410-b5e6-96231b3b80d8
1 file changed