Add BTI landing pads for aarch64

When Branch Target Identifier (BTI) is enabled on aarch64,
any software which run libglvnd will fail with SIGILL, Illegal instruction.
This is because some assembler code misses the BTI landing pads.
See: https://developer.arm.com/documentation/102433/0100/Jump-oriented-programming

"hint #34" is the same thing as "BTI C" landing pad, but keep
compatibility with systems without BTI enabled.

src/GLX/glvnd_genentry.c

@@ -90,6 +90,7 @@ extern char glx_entrypoint_end[];
 
 #define STUB_SIZE 16
 #define STUB_ASM_ARCH(slot) \
+    "hint #34\n" \
     "adrp x16, entrypointFunctions + " slot "*8\n" \
     "ldr x16, [x16, #:lo12:(entrypointFunctions + " slot "*8)]\n" \
     "br x16\n"

src/GLdispatch/vnd-glapi/entry_aarch64_tsd.c

@@ -69,6 +69,7 @@
  * table then does a branch without link to the function address.
  */
 #define STUB_ASM_CODE(slot)                           \
+    "hint #34\n\t"                                    \
     "stp x1, x0, [sp, #-16]!\n\t"                     \
     "adrp x0, :got:_glapi_Current\n\t"                \
     "ldr x0, [x0, #:got_lo12:_glapi_Current]\n\t"     \