[clang][test][RISCV] Add simple litmus test for --print-enabled-extensions

There's some coverage in RISCVISAInfoTest, but it's worth adding a quick
test to ensure nothing happens to the frontend handling of this option.

Author: asb

clang/test/Driver/print-enabled-extensions/riscv-rocket-rv64.c

@@ -0,0 +1,13 @@
+// REQUIRES: riscv-registered-target
+// RUN: %clang --target=riscv64 --print-enabled-extensions -mcpu=rocket-rv64 | FileCheck --strict-whitespace %s
+
+// Simple litmus test to check the frontend handling of this option is
+// enabled.
+
+// CHECK: Extensions enabled for the given RISC-V target 
+// CHECK-EMPTY:
+// CHECK-NEXT: Name                 Version   Description 
+// CHECK-NEXT:     i                    2.1       'I' (Base Integer Instruction Set) 
+// CHECK-NEXT:     zicsr                2.0       'zicsr' (CSRs) 
+// CHECK-NEXT:     zifencei             2.0       'Zifencei' (fence.i) 
+// CHECK-EMPTY: