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

[Wende Tan <twd2.me@gmail.com>]

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@gmail.com>
Signed-off-by: Wende Tan <twd2.me@gmail.com>
---
v2:
- Some textual changes suggested by Nick.
- Drop the changes to `arch/riscv/Makefile`, since the LLVM issue is filed
  and resolved.
- Drop the unnecessary changes to `arch/riscv/kernel/vdso/Makefile`.

v1: https://lore.kernel.org/linux-riscv/20210719205208.1023221-1-twd2.me@gmail.com/
---
 arch/riscv/Kconfig | 4 ++++
 1 file changed, 4 insertions(+)

diff --git a/arch/riscv/Kconfig b/arch/riscv/Kconfig
index 00fd9c548f26..c55f6b95e5af 100644
--- a/arch/riscv/Kconfig
+++ b/arch/riscv/Kconfig
@@ -38,6 +38,10 @@ config RISCV
 	select ARCH_SUPPORTS_ATOMIC_RMW
 	select ARCH_SUPPORTS_DEBUG_PAGEALLOC if MMU
 	select ARCH_SUPPORTS_HUGETLBFS if MMU
+	# LLD >= 14:
+	# - https://github.com/llvm/llvm-project/issues/50505
+	select ARCH_SUPPORTS_LTO_CLANG if LLD_VERSION >= 140000
+	select ARCH_SUPPORTS_LTO_CLANG_THIN if LLD_VERSION >= 140000
 	select ARCH_USE_MEMTEST
 	select ARCH_WANT_DEFAULT_TOPDOWN_MMAP_LAYOUT if MMU
 	select ARCH_WANT_FRAME_POINTERS
-- 
2.25.1