[flang][test] Fix gcc-triple test when default target is not x86_64

By adding --target to the first command.

Test added by #165886.

Author: DavidSpickett

flang/test/Driver/gcc-triple.f90

@@ -2,7 +2,7 @@
 
 !! Test that --gcc-triple option is working as expected.
 
-! RUN: %flang -v --sysroot=%S/Inputs/fedora_39_tree 2>&1 | FileCheck %s --dump-input=always --check-prefix=DEFAULT_TRIPLE
+! RUN: %flang --target=x86_64-linux-gnu -v --sysroot=%S/Inputs/fedora_39_tree 2>&1 | FileCheck %s --dump-input=always --check-prefix=DEFAULT_TRIPLE
 ! DEFAULT_TRIPLE: {{^}}Found candidate GCC installation:
 ! DEFAULT_TRIPLE: fedora_39_tree/usr/lib/gcc/x86_64-linux-gnu/13
 ! DEFAULT_TRIPLE: {{^}}Found candidate GCC installation: