[Ada] Fast implementation of floating-point mathematical functions

[Ada] Fast implementation of floating-point mathematical functions

This adds a package renaming unit to the GNAT hierarchy so as to expose
the underlying implementation of floating-point mathematical functions,
thus also making it possible to use their vector implementation, if any.

The change also contains a small improvement to the Hide_Public_Entities
mechanism in Sem_Ch7 that makes it possible to clear the Is_Public flag
within instances of generic packages that do not have a body.

gcc/ada/

	* Makefile.rtl (GNATRTL_NONTASKING_OBJS): Add g-gfmafu$(objext).
	(SIMD_PATH_TARGET_PAIRS): New variable.
	(TRASYM_DWARF_COMMON_OBJS): Minor tweak.
	(x86-64/Linux): Use SIMD_PATH_TARGET_PAIRS.
	(x32/Linux): Likewise.
	* doc/gnat_rm/the_gnat_library.rst (Generic_Fast_Math_Functions):
	New entry.
	* gnat_rm.texi: Regenerate.
	* impunit.adb (Non_Imp_File_Names_95): Add g-gfmafu.
	* sem_ch7.adb (Has_Referencer): Do not set In_Nested_Instance for
	instances of generic packages that do not have a body.
	* libgnat/a-nalofl__simd.ads: New SIMD-enabled version.
	* libgnat/a-nuaufl__simd.ads: Likewise.
	* libgnat/g-gfmafu.ads: New package renaming unit.