Aarch64: Define WIDEST_HARDWARE_FP_SIZE

The macro is documented like this in the internal manual:

 -- Macro: WIDEST_HARDWARE_FP_SIZE
     A C expression for the size in bits of the widest floating-point
     format supported by the hardware.  If you define this macro, you
     must specify a value less than or equal to mode precision of the
     mode used for C type 'long double' (from hook
     'targetm.c.mode_for_floating_type' with argument
     'TI_LONG_DOUBLE_TYPE').  If you do not define this macro, mode
     precision of the mode used for C type 'long double' is the default.

AArch64 uses 128-bit TFmode for long double but, as far as I know, no FPU
implemented in hardware supports it.

gcc/
	* config/aarch64/aarch64.h (WIDEST_HARDWARE_FP_SIZE): Define to 64.

gcc/testsuite/
	* gnat.dg/specs/size_clause6.ads: New test.