[Driver][test] Ensure --target= for -flto commands

Otherwise if the default target triple has `HasNativeLLVMSupport() ==
false`, the %clang command will exit with code 1 due to
err_drv_no_linker_llvm_support.

The issue is exposed by D156363.

Author: MaskRay

clang/test/Driver/clang_f_opts.c

@@ -114,7 +114,7 @@
 // RUN: %clang -### -S -fprofile-instr-generate -fcoverage-mapping -fno-coverage-mapping %s 2>&1 | FileCheck -check-prefix=CHECK-DISABLE-COVERAGE %s
 // RUN: %clang -### -S -fprofile-remapping-file=foo/bar.txt %s 2>&1 | FileCheck -check-prefix=CHECK-PROFILE-REMAP %s
 // RUN: %clang -### -S -forder-file-instrumentation %s 2>&1 | FileCheck -check-prefix=CHECK-ORDERFILE-INSTR %s
-// RUN: %clang -### -flto -forder-file-instrumentation %s 2>&1 | FileCheck -check-prefix=CHECK-ORDERFILE-INSTR-LTO %s
+// RUN: %clang -### --target=x86_64-linux-gnu -flto -forder-file-instrumentation %s 2>&1 | FileCheck -check-prefix=CHECK-ORDERFILE-INSTR-LTO %s
 // CHECK-PROFILE-GENERATE: "-fprofile-instrument=clang"
 // CHECK-PROFILE-GENERATE-LLVM: "-fprofile-instrument=llvm"
 // CHECK-PROFILE-GENERATE-DIR: "-fprofile-instrument-path=/some/dir{{/|\\\\}}{{.*}}"

clang/test/Driver/lto.c

@@ -16,15 +16,15 @@
 
 // llvm-bc and llvm-ll outputs need to match regular suffixes
 // (unfortunately).
-// RUN: %clang %s -flto -save-temps -### 2> %t
+// RUN: %clang --target=x86_64-linux-gnu %s -flto -save-temps -### 2> %t
 // RUN: FileCheck -check-prefix=CHECK-COMPILELINK-SUFFIXES < %t %s
 //
 // CHECK-COMPILELINK-SUFFIXES: "-o" "{{.*}}lto.i" "-x" "c" "{{.*}}lto.c"
 // CHECK-COMPILELINK-SUFFIXES: "-o" "{{.*}}lto.bc" {{.*}}"{{.*}}lto.i"
 // CHECK-COMPILELINK-SUFFIXES: "-o" "{{.*}}lto.o" {{.*}}"{{.*}}lto.bc"
 // CHECK-COMPILELINK-SUFFIXES: "{{.*}}a.{{(out|exe)}}" {{.*}}"{{.*}}lto.o"
 
-// RUN: %clang %s -flto -S -### 2> %t
+// RUN: %clang --target=x86_64-linux-gnu %s -flto -S -### 2> %t
 // RUN: FileCheck -check-prefix=CHECK-COMPILE-SUFFIXES < %t %s
 //
 // CHECK-COMPILE-SUFFIXES: "-o" "{{.*}}lto.s" "-x" "c" "{{.*}}lto.c"