[DebugInfo][test] XFAIL DebugInfo/Generic/missing-abstract-variable.ll on SPARC

`DebugInfo/Generic/missing-abstract-variable.ll` `FAIL`s on SPARC.  When
`x` is inlined into `a`, the variable `s` is elided completely, so the
corresponding `CHECK`s fail.  Exactly the same happens on RISCV and the
test has already been `XFAIL`ed there.

This patch does the same on SPARC.

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

Differential Revision: https://reviews.llvm.org/D119122