aarch64: Bump AARCH64_APPROX_MODE to 64 bits

aarch64: Bump AARCH64_APPROX_MODE to 64 bits

We now have more than 32 scalar and vector float modes, so the
32-bit AARCH64_APPROX_MODE would invoke UB for some of them.
Bumping to a 64-bit mask fixes that... for now.

Ideally we'd have a static assert to trap this, but logically
it would go at file scope.  I think it would be better to wait
until the switch to C++11, so that we can use static_assert
directly.

2020-02-21  Richard Sandiford  <richard.sandiford@arm.com>

gcc/
	* config/aarch64/aarch64-protos.h (AARCH64_APPROX_MODE): Operate
	on and produce uint64_ts rather than ints.
	(AARCH64_APPROX_NONE, AARCH64_APPROX_ALL): Change to uint64_ts.
	(cpu_approx_modes): Change the fields from unsigned int to uint64_t.