riscv: Only allow LTO with CMODEL_MEDANY

riscv: Only allow LTO with CMODEL_MEDANY

When building with CONFIG_CMODEL_MEDLOW and CONFIG_LTO_CLANG, there is a
series of errors due to some files being unconditionally compiled with
'-mcmodel=medany', mismatching with the rest of the kernel built with
'-mcmodel=medlow':

  ld.lld: error: Function Import: link error: linking module flags 'Code Model': IDs have conflicting values: 'i32 3' from vmlinux.a(init.o at 899908), and 'i32 1' from vmlinux.a(net-traces.o at 1014628)

Only allow LTO to be performed when CONFIG_CMODEL_MEDANY is enabled to
ensure there will be no code model mismatch errors. An alternative
solution would be disabling LTO for the files with a different code
model than the main kernel like some specialized areas of the kernel do
but doing that for individual files is not as sustainable than
forbidding the combination altogether.

Cc: stable@vger.kernel.org
Fixes: 021d23428bdb ("RISC-V: build: Allow LTO to be selected")
Reported-by: kernel test robot <lkp@intel.com>
Closes: https://lore.kernel.org/oe-kbuild-all/202506290255.KBVM83vZ-lkp@intel.com/
Signed-off-by: Nathan Chancellor <nathan@kernel.org>
Link: https://lore.kernel.org/r/20250710-riscv-restrict-lto-to-medany-v1-1-b1dac9871ecf@kernel.org
Signed-off-by: Paul Walmsley <pjw@kernel.org>