Back port PR target/120681

When I implemented the pc-relative support for power10 in GCC, I
disabled using pc-relative support for -mcmodel=large.  At the time, I
didn't want to dig into the issues.  It is now time to allow
-mcmodel=large to generate pc-relative code.

This patch allows -mcmodel=large to use prefixed addressing on power10,
power11, and possibly other future PowerPC processors in addition to
the current -mcmodel=medium support.

This change was back ported from the changes to the master branch on
2026-08-07.

2026-08-27  Michael Meissner  <meissner@linux.ibm.com>

gcc/

	PR target/120681
	* config/rs6000/linux64.h (PCREL_SUPPORTED_BY_OS): Allow large
	code model as well as medium code model.
	* config/rs6000/rs6000.cc (rs6000_option_override_internal):
	Likewise.
	(rs6000_elf_declare_function_name): Don't create the
	local/non-local labels for large code model if we are using
	PC-relative addressing.

gcc/testsuite/

	PR target/120681
	* gcc.target/powerpc/pr120681.c: New test.