Improve generated code for various libgcc2.c routines

libgcc/

	* libgcc2.c (__addvSI3): Use overflow builtins.
	(__addvsi3, __addvDI3 ,__subvSI3, __subvsi3): Likewise.
	(__subvDI3 __mulvSI3, __mulvsi3, __negvSI2): Likewise.
	(__negvsi2, __negvDI2): Likewise.
	(__cmpdi2, __ucmpdi2): Adjust implementation to improve
	generated code.
	* libgcc2.h (__ucmpdi2): Adjust prototype.