AMDGPU: Remove "gws" from the “read-only” target feature list

Author: changpeng

clang/test/CodeGenOpenCL/amdgpu-features-readonly.cl

@@ -0,0 +1,6 @@
+// RUN: %clang_cc1 -triple amdgcn -target-feature +vmem-to-lds-load-insts -o /dev/null %s 2>&1 \
+// RUN:   | FileCheck --check-prefix=READONLY %s
+
+// READONLY: warning: feature flag '+vmem-to-lds-load-insts' is ignored since the feature is read only [-Winvalid-command-line-argument]
+
+kernel void test() {}