rs6000, fix error in unsigned vector float to unsigned int built-in definitions

rs6000, fix error in unsigned vector float to unsigned int built-in definitions

The built-in __builtin_vsx_vunsigned_v2df is supposed to take a vector of
doubles and return a vector of unsigned long long ints.  Similarly
__builtin_vsx_vunsigned_v4sf takes a vector of floats an is supposed to
return a vector of unsinged ints.  The definitions are using the signed
version of the instructions not the unsigned version of the instruction.
The results should also be unsigned.  The built-ins are used by the
overloaded vec_unsigned built-in which has an unsigned result.

Similarly the built-ins __builtin_vsx_vunsignede_v2df and
__builtin_vsx_vunsignedo_v2df are supposed to return an unsigned result.
If the floating point argument is negative, the unsigned result is zero.
The built-ins are used in the overloaded built-in vec_unsignede and
vec_unsignedo respectively.

Add a test cases for a negative floating point arguments for each of the
above built-ins.

gcc/ChangeLog:
	* config/rs6000/rs6000-builtins.def (__builtin_vsx_vunsigned_v2df,
	__builtin_vsx_vunsigned_v4sf, __builtin_vsx_vunsignede_v2df,
	__builtin_vsx_vunsignedo_v2df): Change the result type to unsigned.

gcc/testsuite/ChangeLog:
	* gcc.target/powerpc/builtins-3-runnable.c: Add tests for
	vec_unsignede and vec_unsignedo with negative arguments.