[CodeGen][test] XFAIL CodeGen/Generic/ForceStackAlign.ll on SPARC

`CodeGen/Generic/ForceStackAlign.ll` `FAIL`s on SPARC like this:

  LLVM ERROR: Function "g" required stack re-alignment, but LLVM couldn't
handle it (probably because it has a dynamic alloca).

According to the comments in `llvm/lib/Target/Sparc/SparcFrameLowering.cpp`
(`SparcFrameLowering::emitPrologue`) and `SparcRegisterInfo.cpp`
(`SparcRegisterInfo::canRealignStack`) this isn't going to change any time
soon, so this patch `XFAIL`s the test.

Tested on `sparcv9-sun-solaris2.11`.

Differential Revision: https://reviews.llvm.org/D119119