aarch64: Rename FLAG_AUTO_FP to FLAG_QUIET

aarch64: Rename FLAG_AUTO_FP to FLAG_QUIET

I'd suggested the name "FLAG_AUTO_FP" to mean "automatically derive
FLAG_FP from the mode", i.e. automatically decide whether the function
might read the FPCR or might raise FP exceptions.  However, the flag
currently suppresses that behaviour instead.

This patch renames FLAG_AUTO_FP to FLAG_QUIET.  That's probably not a
great name, but it's also what the SVE code means by "quiet", and is
borrowed from "quiet NaNs".

gcc/
	* config/aarch64/aarch64-builtins.cc (FLAG_AUTO_FP): Rename to...
	(FLAG_QUIET): ...this and update all references.
	* config/aarch64/aarch64-simd-builtins.def: Update all references
	here too.