aarch64/bits/hwcap.h: add HWCAP2_BTI from linux v5.8

hwcap for BTI was missing, see

  linux commit 8ef8f360cf30be12382f89ff48a57fbbd9b31c14
  arm64: Basic Branch Target Identification support

arch/aarch64/bits/hwcap.h

@@ -48,3 +48,4 @@
 #define HWCAP2_BF16		(1 << 14)
 #define HWCAP2_DGH		(1 << 15)
 #define HWCAP2_RNG		(1 << 16)
+#define HWCAP2_BTI		(1 << 17)