clover: Don't unconditionally define cl_khr_fp64

This should be done by the frontend for devices that support this
extension.

Reviewed-by: Francisco Jerez <currojerez@riseup.net>

src/gallium/state_trackers/clover/llvm/invocation.cpp

@@ -203,7 +203,6 @@ namespace {
 
       // clc.h requires that this macro be defined:
       c.getPreprocessorOpts().addMacroDef("cl_clang_storage_class_specifiers");
-      c.getPreprocessorOpts().addMacroDef("cl_khr_fp64");
 
       c.getLangOpts().NoBuiltin = true;
       c.getTargetOpts().Triple = triple;