FROMGIT: kbuild: rust: add CONFIG_RUSTC_LLVM_VERSION

Each version of Rust supports a range of LLVM versions. There are cases where
we want to gate a config on the LLVM version instead of the Rust version.
Normalized cfi integer tags are one example [1].

The invocation of rustc-version is being moved from init/Kconfig to
scripts/Kconfig.include for consistency with cc-version.

Link: https://lore.kernel.org/all/20240925-cfi-norm-kasan-fix-v1-1-0328985cdf33@google.com/ [1]
Signed-off-by: Gary Guo <gary@garyguo.net>
Link: https://lore.kernel.org/r/20241011114040.3900487-1-gary@garyguo.net
[ Added missing `-llvm` to the Usage documentation. - Miguel ]
Signed-off-by: Miguel Ojeda <ojeda@kernel.org>

Bug: 359429865
(cherry picked from commit af0121c2d3
 https://github.com/Rust-for-Linux/linux.git rust-fixes)
Change-Id: I4a54c0c0963504e99583c32b507c8e257301094d
Signed-off-by: Alice Ryhl <aliceryhl@google.com>

scripts/Kconfig.include

@@ -65,6 +65,9 @@ cc-option-bit = $(if-success,$(CC) -Werror $(1) -E -x c /dev/null -o /dev/null,$
 m32-flag := $(cc-option-bit,-m32)
 m64-flag := $(cc-option-bit,-m64)
 
+rustc-version := $(shell,$(srctree)/scripts/rustc-version.sh $(RUSTC))
+rustc-llvm-version := $(shell,$(srctree)/scripts/rustc-llvm-version.sh $(RUSTC))
+
 # $(rustc-option,<flag>)
 # Return y if the Rust compiler supports <flag>, n otherwise
 # Calls to this should be guarded so that they are not evaluated if