RISC-V: build: Allow LTO to be selected

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.

To avoid boot failures in QEMU [2], '-mattr=+c' and '-mattr=+relax'
need to be passed via '-mllvm' to ld.lld, as there appears to be an
issue with LLVM's target-features and LTO [3], which can result in
incorrect relocations to branch targets [4]. Once this is fixed in LLVM,
it can be made conditional on affected ld.lld versions.

Disable LTO for arch/riscv/kernel/pi, as llvm-objcopy expects an ELF
object file when manipulating the files in that subfolder, rather than
LLVM bitcode.

[1] llvm/llvm-project#50505, resolved by LLVM
    commit e63455d5e0e5 ("[MC] Use local MCSubtargetInfo in writeNops")
[2] ClangBuiltLinux#1942
[3] llvm/llvm-project#59350
[4] llvm/llvm-project#65090

Tested-by: Wende Tan <twd2.me@gmail.com>
Signed-off-by: Wende Tan <twd2.me@gmail.com>
Co-developed-by: Nathan Chancellor <nathan@kernel.org>
Signed-off-by: Nathan Chancellor <nathan@kernel.org>
Reviewed-by: Conor Dooley <conor.dooley@microchip.com>
Link: https://lore.kernel.org/r/20231017-riscv-lto-v4-1-e7810b24e805@kernel.org
Signed-off-by: Palmer Dabbelt <palmer@rivosinc.com>

arch/riscv/Kconfig

@@ -38,6 +38,9 @@ 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_SUPPORTS_PAGE_TABLE_CHECK if MMU
 	select ARCH_USE_MEMTEST
 	select ARCH_USE_QUEUED_RWLOCKS

arch/riscv/Makefile

@@ -48,6 +48,11 @@ ifndef CONFIG_AS_IS_LLVM
 	KBUILD_CFLAGS += -Wa,-mno-relax
 	KBUILD_AFLAGS += -Wa,-mno-relax
 endif
+# LLVM has an issue with target-features and LTO: https://github.com/llvm/llvm-project/issues/59350
+# Ensure it is aware of linker relaxation with LTO, otherwise relocations may
+# be incorrect: https://github.com/llvm/llvm-project/issues/65090
+else ifeq ($(CONFIG_LTO_CLANG),y)
+	KBUILD_LDFLAGS += -mllvm -mattr=+c -mllvm -mattr=+relax
 endif
 endif