kbuild: rust: avoid errors with old rustcs without LLVM patch version

Some old versions of `rustc` did not report the LLVM version without
the patch version, e.g.:

    $ rustc --version --verbose
    rustc 1.48.0 (7eac88abb 2020-11-16)
    binary: rustc
    commit-hash: 7eac88abb2e57e752f3302f02be5f3ce3d7adfb4
    commit-date: 2020-11-16
    host: x86_64-unknown-linux-gnu
    release: 1.48.0
    LLVM version: 11.0

Which would make the new `scripts/rustc-llvm-version.sh` fail and,
in turn, the build:

    $ make LLVM=1
      SYNC    include/config/auto.conf.cmd
    ./scripts/rustc-llvm-version.sh: 13: arithmetic expression: expecting primary: "10000 * 10 + 100 * 0 + "
    init/Kconfig:83: syntax error
    init/Kconfig:83: invalid statement
    make[3]: *** [scripts/kconfig/Makefile:85: syncconfig] Error 1
    make[2]: *** [Makefile:679: syncconfig] Error 2
    make[1]: *** [/home/cam/linux/Makefile:780: include/config/auto.conf.cmd] Error 2
    make: *** [Makefile:224: __sub-make] Error 2

Since we do not need to support such binaries, we can avoid adding logic
for computing `rustc`'s LLVM version for those old binaries.

Thus, instead, just make the match stricter.

Other `rustc` binaries (even newer) did not report the LLVM version at
all, but that was fine, since it would not match "LLVM", e.g.:

    $ rustc --version --verbose
    rustc 1.49.0 (e1884a8e3 2020-12-29)
    binary: rustc
    commit-hash: e1884a8e3c3e813aada8254edfa120e85bf5ffca
    commit-date: 2020-12-29
    host: x86_64-unknown-linux-gnu
    release: 1.49.0

Cc: Thorsten Leemhuis <[email protected]>
Cc: Gary Guo <[email protected]>
Reported-by: Cameron MacPherson <[email protected]>
Closes: https://bugzilla.kernel.org/show_bug.cgi?id=219423
Fixes: af0121c ("kbuild: rust: add `CONFIG_RUSTC_LLVM_VERSION`")
Tested-by: Cameron MacPherson <[email protected]>
Reviewed-by: Nathan Chancellor <[email protected]>
Tested-by: Nathan Chancellor <[email protected]>
Link: https://lore.kernel.org/r/[email protected]
Signed-off-by: Miguel Ojeda <[email protected]>

scripts/rustc-llvm-version.sh

@@ -13,7 +13,7 @@ get_canonical_version()
 	echo $((10000 * $1 + 100 * $2 + $3))
 }
 
-if output=$("$@" --version --verbose 2>/dev/null | grep LLVM); then
+if output=$("$@" --version --verbose 2>/dev/null | grep -E 'LLVM.*[0-9]+\.[0-9]+\.[0-9]+'); then
 	set -- $output
 	get_canonical_version $3
 else