[flang] Add Solaris/x86 support to Optimizer/CodeGen/Target.cpp

When testing LLVM 15.0.0 rc1 on Solaris, I found that 50+ flang tests
`FAIL`ed with

  error:
/vol/llvm/src/llvm-project/local/flang/lib/Optimizer/CodeGen/Target.cpp:310:
not yet implemented: target not implemented

This patch fixes that for Solaris/x86, where the fix is trivial (just
handling it like the other x86 OSes).

Tested on `amd64-pc-solaris2.11`; only a single failure remains now.

Differential Revision: https://reviews.llvm.org/D131054