* libc/machine/arm/strcmp.S: Use local labels.
	* libc/include/cdefs.h (__has_extension): New define.
	(__has_feature): Likewise.
	(__has_include): Likewise.
	(__has_builtin): Likewise.
	* libc/include/stdatomic.h: New.
Fix typos
	* libc/stdlib/gdtoa-gethex.c (__hexdig): Constify.
	(hexdig_init): Remove.
	(__hexdig_fun): New function.
	hexdig_init, added __hexdig_fun
	(gethex): Call __get_hexdig macro rather than hexdig.
	* libc/stdlib/gdtoa-hexnan.c (hexnan): Constify fpi argument.
	Call __get_hexdig macro rather than hexdig.
	* libc/stdlib/ldtoa.c: Throughout constify functions arguments where
	required by constifying the following arrays.
	(ezero): Constify.
	(eone): Constify.
	(ermsg): Constify.
	(etens): Constify.
	(emtens): Constify.
	(nan113): Constify.
	(nan64): Constify.
	(nan53): Constify.
	(nan24): Constify.
	* libc/stdlib/mprec.h (__get_hexdig): Define.
	(gethex): Constify args in declaration where appropriate.
	(hexnan): Ditto.
	(hexdig_init): Remove declaration.
	(__hexdig_fun): Declare.
	* libc/stdlib/strtod.c (fpi): Constify.
	(fpinan): Constify.
	* README: Add description for NEWLIB's feature customizing
	configuration options.