[AArch64] Only build & test pauth code for LP64

[AArch64] Only build & test pauth code for LP64

gcc/
	* config/aarch64/aarch64-builtins.c (aarch64_init_builtins): Register
	register pauth builtins for LP64 only.

libgcc/
	* config/aarch64/aarch64-unwind.h: Empty this file on ILP32.
	* unwind-dw2.c (execute_cfa_program):  Only multiplexing
	DW_CFA_GNU_window_save for AArch64 and LP64.

gcc/testsuite/
	* testsuite/gcc.target/aarch64/return_address_sign_1.c: Enable on LP64
	only.
	* testsuite/gcc.target/aarch64/return_address_sign_2.c: Likewise.
	* testsuite/gcc.target/aarch64/return_address_sign_3.c: Likewise.

From-SVN: r244732