Define the __ARM_FEATURE_PAC_DEFAULT feature test
macro when PAC-RET branch protection is enabled.

2020-07-13  Szabolcs Nagy  <szabolcs.nagy@arm.com>

gcc/ChangeLog:

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

(cherry picked from commit a1faa8e2470b33e92f6274804bf7941fbb6e2d38)