aarch64: define struct user_pac_mask if needed

aarch64: define struct user_pac_mask if needed

On Aarch64, Linux is using Pointer Authentication Code (PAC) for pointer
authentication.[1] The struct "user_pac_mask" has been part of the Linux
kernel since version 5.0 as part of this feature. However, older kernels
do not define it.

Therefore, we want to check if the definition is present in the kernel
headers and provide one if it isn't. This ensures two things:

* elfutils will continue to compile against kernel headers from 4.x
* binaries built against older kernel headers will still be fully
  functional if used on a newer system

For reference, the build error that is being avoided looks like this:

[...]
  CC       aarch64_initreg.o
aarch64_initreg.c: In function 'aarch64_set_initial_registers_tid':
aarch64_initreg.c:61:24: error: storage size of 'pac_mask' isn't known
   struct user_pac_mask pac_mask;
                        ^~~~~~~~
aarch64_initreg.c:61:24: warning: unused variable 'pac_mask' [-Wunused-variable]
make[4]: *** [Makefile:831: aarch64_initreg.o] Error 1
make[3]: *** [Makefile:547: all-recursive] Error 1
make[2]: *** [Makefile:463: all] Error 2

[1] https://docs.kernel.org/arch/arm64/pointer-authentication.html

https://sourceware.org/bugzilla/show_bug.cgi?id=32684

Fixes: 64e3b451ad2c ("aarch64: extend dwfl_thread_state_registers to handle PAC")
Signed-off-by: Markus Mayer <mmayer@broadcom.com>