libgcc: xtensa: use built-in configuration

libgcc: xtensa: use built-in configuration

Now that gcc provides __XCHAL_* definitions use them instead of XCHAL_*
definitions from the include/xtensa-config.h. That makes libgcc
dynamically configurable for the target xtensa core.

libgcc/
	* config/xtensa/crti.S (xtensa-config.h): Replace #inlcude with
	xtensa-config-builtin.h.
	* config/xtensa/crtn.S: Likewise.
	* config/xtensa/lib1funcs.S: Likewise.
	* config/xtensa/lib2funcs.S: Likewise.
	* config/xtensa/xtensa-config-builtin.h: New File.