Update crtbegin.c and crt*.S to support Armv8.5-A BTI

These files are linked to all ELF files therefore they must support BTI.

Test: Tested on FVP with BTI enabled using a patched clang.

Co-authored-by: Gabor Kertesz <gabor.kertesz@arm.com>
Co-authored-by: Daniel Kiss   <daniel.kiss@arm.com>
Co-authored-by: Tamas Petz    <tamas.petz@arm.com>

Change-Id: If5df0722e649bcdb8c4afb0531831dff42103c9c
diff --git a/libc/arch-common/bionic/crtbrand.S b/libc/arch-common/bionic/crtbrand.S
index 34d6480..3d80d73 100644
--- a/libc/arch-common/bionic/crtbrand.S
+++ b/libc/arch-common/bionic/crtbrand.S
@@ -26,6 +26,12 @@
  * SUCH DAMAGE.
  */
 
+#if defined(__aarch64__)
+#include <private/bionic_asm_arm64.h>
+
+__bionic_asm_custom_note_gnu_section()
+#endif
+
   .section .note.android.ident,"a",%note
   .balign 4
   .type abitag, %object