[flang] Add Sparc support to Optimizer/CodeGen/Target.cpp

As described in Issue #57642, `flang` currently lacks SPARC support in
`Optimizer/CodeGen/Target.cpp`, which causes a considerable number of tests
to `FAIL` with

  error: flang/lib/Optimizer/CodeGen/Target.cpp:310: not yet implemented:
target not implemented

This patch fixes this by following GCC`s documentation of the ABI described
in the Issue.

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

Differential Revision: https://reviews.llvm.org/D133561