Ignore vendor name in Clang version number.

Ignore vendor name in Clang version number.

For example, FreeBSD prepends "FreeBSD" to version string, e.g.,

FreeBSD clang version 11.0.0 (git@github.com:llvm/llvm-project.git llvmorg-11.0.0-rc2-0-g414f32a9e86)
Target: x86_64-unknown-freebsd13.0
Thread model: posix
InstalledDir: /usr/bin

This prevented us from properly detecting AVX support, etc.

CLA: trivial

Reviewed-by: Richard Levitte <levitte@openssl.org>
Reviewed-by: Paul Dale <paul.dale@oracle.com>
Reviewed-by: Ben Kaduk <kaduk@mit.edu>
(Merged from https://github.com/openssl/openssl/pull/12725)