Enable watchpoint support for PowerPC

PowerPC supports data watchpoint through DAWR register.
The invalid watchpoint modes referenced from
PowerISA Book III v3.1B, Section 5.4.4 and 10.4

Signed-off-by: Abhishek Dubey <adubey@linux.ibm.com>

src/arch/ppc64.cpp

@@ -129,8 +129,13 @@ std::string name()
 
 std::vector<std::string> invalid_watchpoint_modes()
 {
-  throw std::runtime_error(
-      "Watchpoints are not supported on this architecture");
+  // See PowerISA Book III v3.1B, Section 5.4.4 and 10.4
+  return std::vector<std::string>{
+    "x",
+    "rx",
+    "wx",
+    "rwx",
+  };
 }
 
 } // namespace arch