tests/tcg/aarch64: Fix compilation parameters for pauth-%

tests/tcg/aarch64: Fix compilation parameters for pauth-%

We were incorrectly requiring ARMv8.4 support for the pauth
tests, but Pointer Authentication is an ARMv8.3 extension.
Further, hiding the required architecture within asm() is
not correct.

Correct the architecture version requested, and specify it
in the cflags of the (cross-) compiler rather than in the asm.

Signed-off-by: Richard Henderson <[email protected]>
Reviewed-by: Philippe Mathieu-Daudé <[email protected]>
Message-id: 20200116230809[email protected]
[PMM: tweaked commit message]
Signed-off-by: Peter Maydell <[email protected]>