RS6000 Add support to print vector register contents as float128

This patch adds a floating point 128-bit composite field to the vsx
register type.  When printing the register with p/f the float128 field will
be printed as a 128-bit floating point value.  A test case to verify the new
vsx register field is visible and correctly prints out the value of a 128-bit
floating point value is also added.

gdb/ChangeLog:

	* rs6000-tdep.c (rs6000_builtin_type_vec128): Add t_float128 variable.
	(rs6000_builtin_type_vec128): Add append_composite_type_field for
	float128.

gdb/testsuite/ChangeLog:

	* gdb.arch/vsx-vsr-float128.c: New test file.
	* gdb.arch/vsx-vsr-float128.exp: New expect file.