[PATCH v2] RISC-V: build: Allow LTO to be selected

[Palmer Dabbelt]

> Allow LTO to be selected for RISC-V, only when LLD >= 14, since there is an
> issue [1] in prior LLD versions that prevents LLD to generate proper
> machine code for RISC-V when writing `nop`s.
> 
> I have tested enabling LTO for `defconfig`. The LLD took ~2m21s and ~3GiB
> on our Intel Xeon Gold 6140 server and produced an 18MiB Image. The image
> can boot to shell using an archriscv rootfs on QEMU.
> 
> I have also tested it for `allyesconfig` without COMPILE_TEST, FTRACE,
> KASAN, and GCOV. The LLD took ~7h03m and ~335GiB on the server,
> successfully producing a 1.7GiB Image. Unfortunately, we cannot boot this
> image because the `create_kernel_page_table()` -> `alloc_pmd_early()` ->
> `BUG_ON()` logic limits the image to be < 1GiB. Maybe we can fix it in a
> separate patch further.
> 
> [1] https://github.com/llvm/llvm-project/issues/50505, resolved by LLVM
>     commit e63455d5e0e5 ("[MC] Use local MCSubtargetInfo in writeNops")
> 
> Tested-by: Wende Tan <twd2.me at gmail.com>
> Signed-off-by: Wende Tan <twd2.me at gmail.com>

Sorry for missing this, the v2 never made it to my inbox.  Not sure 
exactly what happened, but an off-list ping made it through.  I've put 
this on for-next, I don't have any way to test it because I don't have 
clang setup yet.