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/[e-mail]/ [1]
Signed-off-by: Gary Guo <[e-mail]>
Link: https://lore.kernel.org/r/[e-mail]
[ Added missing `-llvm` to the Usage documentation. - Miguel ]
Signed-off-by: Miguel Ojeda <[e-mail]>

init/Kconfig

···
 
 config RUSTC_VERSION
 	int
-	default $(shell,$(srctree)/scripts/rustc-version.sh $(RUSTC))
+	default $(rustc-version)
 	help
 	  It does not depend on `RUST` since that one may need to use the version
 	  in a `depends on`.
···
 
 	  In particular, the Makefile target 'rustavailable' is useful to check
 	  why the Rust toolchain is not being detected.
+
+config RUSTC_LLVM_VERSION
+	int
+	default $(rustc-llvm-version)
 
 config CC_CAN_LINK
 	bool

scripts/Kconfig.include

···
 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

scripts/rustc-llvm-version.sh

···
+#!/bin/sh
+# SPDX-License-Identifier: GPL-2.0
+#
+# Usage: $ ./rustc-llvm-version.sh rustc
+#
+# Print the LLVM version that the Rust compiler uses in a 6 digit form.
+
+# Convert the version string x.y.z to a canonical up-to-6-digits form.
+get_canonical_version()
+{
+	IFS=.
+	set -- $1
+	echo $((10000 * $1 + 100 * $2 + $3))
+}
+
+if output=$("$@" --version --verbose 2>/dev/null | grep LLVM); then
+	set -- $output
+	get_canonical_version $3
+else
+	echo 0
+	exit 1
+fi