[Flang] Remove some incorrect UNSUPPORTED clauses

[probinson]

Summary

These pass for me, running on Windows with a Darwin default target triple.

[awarzynski]

LGTM, thanks!

flang/test/Driver/code-gen-x86.f90

 ! Test -emit-obj (X86)
 
 ! REQUIRES: x86-registered-target, aarch64-registered-target
-! UNSUPPORTED: darwin, macos
 
 ! RUN: rm -f %t.o
 ! RUN: %flang_fc1 -triple x86_64-unknown-linux-gnu -emit-obj %s -o %t.o
 ! RUN: llvm-objdump --triple x86_64-unknown-linux-gnu --disassemble-all %t.o | FileCheck %s --check-prefix=CORRECT_TRIPLE
 ! RUN: rm -f %t.o
 ! RUN: %flang --target=x86_64-unknown-linux-gnu -c %s -o %t.o
 ! RUN: llvm-objdump --triple x86_64-unknown-linux-gnu --disassemble-all %t.o | FileCheck %s --check-prefix=CORRECT_TRIPLE
 

flang/test/Fir/target-rewrite-complex16.fir

 // RUN: fir-opt --target-rewrite="target=x86_64-unknown-linux-gnu" %s | FileCheck %s
-// UNSUPPORTED: darwin
 
 // Test that we rewrite the signature and body of a func.function that returns a
 // complex<16>.
 func.func @returncomplex16() -> !fir.complex<16> {
   %1 = fir.undefined !fir.complex<16>
   %2 = arith.constant 2.0 : f128
   %3 = fir.convert %2 : (f128) -> !fir.real<16>
   %c0 = arith.constant 0 : i32