Remove double underscore from the macro name

Make the user-exposed macro consistent with naming standards
for C++ libraries, ours in particular.

https://eel.is/c++draft/lex.name

Signed-off-by: Artem Gindinson <artem.gindinson@intel.com>

Author: Artem Gindinson

sycl/include/CL/sycl/ONEAPI/atomic_ref.hpp

@@ -455,7 +455,7 @@ class atomic_ref_impl<
               memory_scope scope = default_scope) const noexcept {
 // TODO: Remove the "native atomics" macro check once implemented for all
 // backends
-#if defined(__SYCL_DEVICE_ONLY__) && defined(__SYCL_USE_NATIVE_FP_ATOMICS__)
+#if defined(__SYCL_DEVICE_ONLY__) && defined(SYCL_USE_NATIVE_FP_ATOMICS)
     return detail::spirv::AtomicFAdd(ptr, scope, order, operand);
 #else
     auto load_order = detail::getLoadOrder(order);
@@ -478,7 +478,7 @@ class atomic_ref_impl<
               memory_scope scope = default_scope) const noexcept {
 // TODO: Remove the "native atomics" macro check once implemented for all
 // backends
-#if defined(__SYCL_DEVICE_ONLY__) && defined(__SYCL_USE_NATIVE_FP_ATOMICS__)
+#if defined(__SYCL_DEVICE_ONLY__) && defined(SYCL_USE_NATIVE_FP_ATOMICS)
     return detail::spirv::AtomicFAdd(ptr, scope, order, -operand);
 #else
     auto load_order = detail::getLoadOrder(order);

sycl/test/atomic_ref/add.cpp

@@ -2,7 +2,7 @@
 // mark
 // UNSUPPORTED: cuda
 
-// RUN: %clangxx -fsycl -fsycl-unnamed-lambda -D__SYCL_USE_NATIVE_FP_ATOMICS__ \
+// RUN: %clangxx -fsycl -fsycl-unnamed-lambda -DSYCL_USE_NATIVE_FP_ATOMICS \
 // RUN:  -fsycl-device-only -S %s -o - | FileCheck %s --check-prefix=CHECK-LLVM
 // RUN: %clangxx -fsycl -fsycl-unnamed-lambda -fsycl-device-only -S %s -o - \
 // RUN: | FileCheck %s --check-prefix=CHECK-LLVM-EMU

sycl/test/atomic_ref/sub.cpp

@@ -2,7 +2,7 @@
 // mark
 // UNSUPPORTED: cuda
 
-// RUN: %clangxx -fsycl -fsycl-unnamed-lambda -D__SYCL_USE_NATIVE_FP_ATOMICS__ \
+// RUN: %clangxx -fsycl -fsycl-unnamed-lambda -DSYCL_USE_NATIVE_FP_ATOMICS \
 // RUN:  -fsycl-device-only -S %s -o - | FileCheck %s --check-prefix=CHECK-LLVM
 // RUN: %clangxx -fsycl -fsycl-unnamed-lambda -fsycl-device-only -S %s -o - \
 // RUN: | FileCheck %s --check-prefix=CHECK-LLVM-EMU