[SYCL][Doc] Add feature test macro to sycl_ext_oneapi_bfloat16 (#19830)

The sycl_ext_oneapi_bfloat16 extension was missing a feature test macro
and the related versioning table. This commit adds these to the
extension and adds the definition of the macro to the definitions file.

Closes #18638.

Signed-off-by: Larsen, Steffen <[email protected]>

Author: steffenlarsen

sycl/doc/extensions/supported/sycl_ext_oneapi_bfloat16.asciidoc

@@ -91,6 +91,24 @@ in software.
 
 == Specification
 
+=== Feature test macro
+
+This extension provides a feature-test macro as described in the core SYCL
+specification.  An implementation supporting this extension must predefine the
+macro `SYCL_EXT_ONEAPI_BFLOAT16` to one of the values defined in the table
+below.  Applications can test for the existence of this macro to determine if
+the implementation supports this feature, or applications can test the macro's
+value to determine which of the extension's features the implementation
+supports.
+
+[%header,cols="1,5"]
+|===
+|Value
+|Description
+
+|1
+|Initial version of this extension.
+|===
 
 === New `bfloat16` class
 

sycl/source/feature_test.hpp.in

@@ -56,6 +56,7 @@ inline namespace _V1 {
 #define SYCL_EXT_ONEAPI_SUB_GROUP 1
 #define SYCL_EXT_ONEAPI_PROPERTIES 1
 #define SYCL_EXT_ONEAPI_NATIVE_MATH 1
+#define SYCL_EXT_ONEAPI_BFLOAT16 1
 #define SYCL_EXT_ONEAPI_BFLOAT16_MATH_FUNCTIONS 1
 #define SYCL_EXT_ONEAPI_COMPOSITE_DEVICE 1
 #define SYCL_EXT_INTEL_DATAFLOW_PIPES 1