[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