[flang] Fix gcc-triple.f90 test on Darwin (#171690)

On Darwin an explicit Linux target must be set to make
`Selected GCC installation:` appear in the output.

Author: luporl

flang/test/Driver/gcc-triple.f90

@@ -10,9 +10,9 @@
 ! DEFAULT_TRIPLE: {{^}}Selected GCC installation:
 ! DEFAULT_TRIPLE: fedora_39_tree/usr/lib/gcc/x86_64-linux-gnu/13
 
-! RUN: %flang -v --sysroot=%S/Inputs/fedora_39_tree --gcc-triple=x86_64-redhat-linux 2>&1 | FileCheck %s --check-prefix=TRIPLE_EXISTS
+! RUN: %flang --target=x86_64-redhat-linux-gnu -v --sysroot=%S/Inputs/fedora_39_tree --gcc-triple=x86_64-redhat-linux 2>&1 | FileCheck %s --check-prefix=TRIPLE_EXISTS
 ! TRIPLE_EXISTS: {{^}}Selected GCC installation:
 ! TRIPLE_EXISTS: fedora_39_tree/usr/lib/gcc/x86_64-redhat-linux/13
 
-! RUN: %flang -v --sysroot=%S/Inputs/fedora_39_tree --gcc-triple=x86_64-foo-linux 2>&1 | FileCheck %s --check-prefix=TRIPLE_DOES_NOT_EXISTS
-! TRIPLE_DOES_NOT_EXISTS-NOT: x86_64-foo-linux
+! RUN: %flang --target=x86_64-redhat-linux-gnu -v --sysroot=%S/Inputs/fedora_39_tree --gcc-triple=x86_64-foo-linux 2>&1 | FileCheck %s --check-prefix=TRIPLE_DOES_NOT_EXISTS
+! TRIPLE_DOES_NOT_EXISTS-NOT: x86_64-foo-linux