LoongArch: Handle jump tables options for RUST

When compiling with LLVM and CONFIG_RUST is set, there exist objtool
warnings in rust/core.o and rust/kernel.o, like this:

    rust/core.o: warning: objtool:
_RNvXs1_NtNtCs5QSdWC790r4_4core5ascii10ascii_charNtB5_9AsciiCharNtNtB9_3fmt5Debug3fmt+0x54:
sibling call from callable instruction with modified stack frame

For this special case, the related object file shows that there is no
generated relocation section '.rela.discard.tablejump_annotate' for the
table jump instruction jirl, thus objtool can not know that what is the
actual destination address.

If rustc has the option "-Cllvm-args=--loongarch-annotate-tablejump",
pass the option to enable jump tables for objtool, otherwise it should
pass "-Zno-jump-tables" to keep compatibility with older rustc.

How to test:

  $ rustup component add rust-src
  $ make LLVM=1 rustavailable
  $ make ARCH=loongarch LLVM=1 clean defconfig
  $ scripts/config -d MODVERSIONS \
    -e RUST -e SAMPLES -e SAMPLES_RUST \
    -e SAMPLE_RUST_CONFIGFS -e SAMPLE_RUST_MINIMAL \
    -e SAMPLE_RUST_MISC_DEVICE -e SAMPLE_RUST_PRINT \
    -e SAMPLE_RUST_DMA -e SAMPLE_RUST_DRIVER_PCI \
    -e SAMPLE_RUST_DRIVER_PLATFORM -e SAMPLE_RUST_DRIVER_FAUX \
    -e SAMPLE_RUST_DRIVER_AUXILIARY -e SAMPLE_RUST_HOSTPROGS
  $ make ARCH=loongarch LLVM=1 olddefconfig all

Cc: [email protected]
Acked-by: Miguel Ojeda <[email protected]>
Reported-by: Miguel Ojeda <[email protected]>
Closes: https://lore.kernel.org/rust-for-linux/CANiq72mNeCuPkCDrG2db3w=AX+O-zYrfprisDPmRac_qh65Dmg@mail.gmail.com/
Suggested-by: WANG Rui <[email protected]>
Signed-off-by: Tiezhu Yang <[email protected]>
Signed-off-by: Huacai Chen <[email protected]>

Author: seehearfeel

arch/loongarch/Kconfig

@@ -298,6 +298,10 @@ config AS_HAS_LVZ_EXTENSION
 config CC_HAS_ANNOTATE_TABLEJUMP
 	def_bool $(cc-option,-mannotate-tablejump)
 
+config RUSTC_HAS_ANNOTATE_TABLEJUMP
+	depends on RUST
+	def_bool $(rustc-option,-Cllvm-args=--loongarch-annotate-tablejump)
+
 menu "Kernel type and options"
 
 source "kernel/Kconfig.hz"

arch/loongarch/Makefile

@@ -106,6 +106,11 @@ KBUILD_CFLAGS			+= -mannotate-tablejump
 else
 KBUILD_CFLAGS			+= -fno-jump-tables # keep compatibility with older compilers
 endif
+ifdef CONFIG_RUSTC_HAS_ANNOTATE_TABLEJUMP
+KBUILD_RUSTFLAGS		+= -Cllvm-args=--loongarch-annotate-tablejump
+else
+KBUILD_RUSTFLAGS		+= -Zno-jump-tables # keep compatibility with older compilers
+endif
 ifdef CONFIG_LTO_CLANG
 # The annotate-tablejump option can not be passed to LLVM backend when LTO is enabled.
 # Ensure it is aware of linker with LTO, '--loongarch-annotate-tablejump' also needs to