docs: Update IR/SIL testing section to include powerpc64{le}

Author: ruscur

docs/Testing.rst

@@ -377,10 +377,12 @@ use this pattern::
   // RUN: %target-swift-frontend ... | FileCheck --check-prefix=CHECK --check-prefix=CHECK-%target-cpu %s
 
   // CHECK: common line
-  // CHECK-i386:   only for i386
-  // CHECK-x86_64: only for x86_64
-  // CHECK-armv7:  only for armv7
-  // CHECK-arm64:  only for arm64
+  // CHECK-i386:        only for i386
+  // CHECK-x86_64:      only for x86_64
+  // CHECK-armv7:       only for armv7
+  // CHECK-arm64:       only for arm64
+  // CHECK-powerpc64:   only for powerpc64
+  // CHECK-powerpc64le: only for powerpc64le
 
 Features for ``REQUIRES`` and ``XFAIL``
 ^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^