arm64: Mark the armv8 crc32c as supporting BTI

This is built into a test so needs the BTI elf note for the rest of the
test to be built with BTI support enabled.

As the assembly uses the ENTRY macro it already supports BTI, so mark
it as such.

Sponsored by:	Arm Ltd
Differential Revision:	https://reviews.freebsd.org/D42595

sys/libkern/arm64/crc32c_armv8.S

@@ -26,6 +26,8 @@
  */
 
 #include <machine/asm.h>
+#include <sys/elf_common.h>
+
 .arch_extension crc
 
 /*
@@ -76,3 +78,5 @@ last_byte:
 end:
 	ret
 END(armv8_crc32c)
+
+GNU_PROPERTY_AARCH64_FEATURE_1_NOTE(GNU_PROPERTY_AARCH64_FEATURE_1_VAL)