[Driver][test] Fix dragonfly.c after #69095

If DEFAULT_SYSROOT is not empty, the /usr/lib/gcc80 path may have
leading path components
`getFilePaths().push_back(concat(getDriver().SysRoot, "/usr/lib/gcc80"))`

Author: MaskRay

clang/test/Driver/dragonfly.c

@@ -1,5 +1,4 @@
-// RUN: %clang --target=x86_64-pc-dragonfly -### %s 2> %t.log
-// RUN: FileCheck -input-file %t.log %s
+// RUN: %clang --target=x86_64-pc-dragonfly --sysroot= -### %s 2>&1 | FileCheck %s
 
 // CHECK: "-cc1" "-triple" "x86_64-pc-dragonfly"
 // CHECK: ld{{.*}}" "--eh-frame-hdr" "-dynamic-linker" "/usr/libexec/ld-elf.so.{{.*}}" "--hash-style=gnu" "--enable-new-dtags" "-o" "a.out" "{{.*}}crt1.o" "{{.*}}crti.o" "{{.*}}crtbegin.o" "-L{{.*}}/../lib" "-L/usr/lib" "-L/usr/lib/gcc80" "{{.*}}.o" "-rpath" "{{.*}}gcc80{{.*}}" "-lc" "-lgcc" "{{.*}}crtend.o" "{{.*}}crtn.o"