aarch64: Add missing ACLE support for BTI

Define the __ARM_FEATURE_BTI_DEFAULT feature test
macro when BTI branch protection is enabled.

2020-07-09  Szabolcs Nagy  <szabolcs.nagy@arm.com>

gcc/ChangeLog:

	* config/aarch64/aarch64-c.c (aarch64_update_cpp_builtins): Add
	__ARM_FEATURE_BTI_DEFAULT support.

(cherry picked from commit 63b6808e69699ba576492efa29d92b626cc26eba)

gcc/config/aarch64/aarch64-c.c

@@ -178,6 +178,9 @@ aarch64_update_cpp_builtins (cpp_reader *pfile)
   aarch64_def_or_undef (TARGET_RNG, "__ARM_FEATURE_RNG", pfile);
   aarch64_def_or_undef (TARGET_MEMTAG, "__ARM_FEATURE_MEMORY_TAGGING", pfile);
 
+  aarch64_def_or_undef (aarch64_bti_enabled (),
+			"__ARM_FEATURE_BTI_DEFAULT", pfile);
+
   aarch64_def_or_undef (TARGET_I8MM, "__ARM_FEATURE_MATMUL_INT8", pfile);
   aarch64_def_or_undef (TARGET_BF16_SIMD,
 			"__ARM_FEATURE_BF16_VECTOR_ARITHMETIC", pfile);