d: Compile-time reflection for supported built-ins (PR101127)

d: Compile-time reflection for supported built-ins (PR101127)

In order to allow user-code to determine whether a back-end builtin is
available without error, LANG_HOOKS_BUILTIN_FUNCTION_EXT_SCOPE has been
defined to delay putting back-end builtin functions until the ISA that
defines them has been declared.

However in D, there is no global namespace.  All builtins get pushed
into the `gcc.builtins' module, which is constructed during the semantic
analysis pass, which has already finished by the time target attributes
are evaluated.  So builtins are not pushed by the new langhook because
they would be ultimately ignored.  Builtins exposed to D code then can
now only be altered by the command-line.

	PR d/101127

gcc/d/ChangeLog:

	* d-builtins.cc (d_builtin_function_ext_scope): New function.
	* d-lang.cc (LANG_HOOKS_BUILTIN_FUNCTION_EXT_SCOPE): Define.
	* d-tree.h (d_builtin_function_ext_scope): Declare.

gcc/testsuite/ChangeLog:

	* gdc.dg/pr101127a.d: New test.
	* gdc.dg/pr101127b.d: New test.