rs6000: Fix PTImode attribute handling [PR106895]

PTImode is used to generate even/odd register pairs for 128-bit values.
When PTImode is specified via a type attribute, compilation fails
because no internal type exists to represent this mode.

Introduce signed and unsigned PTImode internal builtin types to handle
PTImode. These __pti_internal types are not documented, as they are not
intended for direct user use.

2026-06-04  Jeevitha Palanisamy  <jeevitha@linux.ibm.com>

gcc/
	PR target/106895
	* config/rs6000/rs6000.h (enum rs6000_builtin_type_index): Add
	RS6000_BTI_INTPTI and RS6000_BTI_UINTPTI.
	(intPTI_type_internal_node, uintPTI_type_internal_node): New
	PTImode type macros.
	* config/rs6000/rs6000-builtin.cc (rs6000_init_builtins): Register
	signed and unsigned PTImode internal builtin types.
	* config/rs6000/sync.md (trunctipti2): New splitter.
	(extendptiti2): Likewise.
	(zero_extendptiti2): Likewise.

gcc/testsuite/
	PR target/106895
	* gcc.target/powerpc/pr106895-1.c: New test.
	* gcc.target/powerpc/pr106895-2.c: New test.