aarch64: Use a global map to detect duplicated overloads [PR112989]

aarch64: Use a global map to detect duplicated overloads [PR112989]

As explained in the covering note to the previous patch,
the fact that aarch64-sve-* is now used for multiple header
files means that function_builder::add_overloaded_function
now needs to use a global map to detect duplicated overload
functions, instead of the member variable that it used previously.

gcc/
	PR target/112989
	* config/aarch64/aarch64-sve-builtins.h
	(function_builder::m_overload_names): Replace with...
	* config/aarch64/aarch64-sve-builtins.cc (overload_names): ...this
	new global.
	(add_overloaded_function): Update accordingly, using get_identifier
	to get a GGC-friendly record of the name.