x86: make embedded endbr64 check compatible with older GNU grep

x86: make embedded endbr64 check compatible with older GNU grep

With version 2.7 I'm observing support for binary searches, but
unreliable results: Only a subset of the supposed matches is actually
reported; for our pattern I've never observed any match. This same
version works fine when handing it a Perl regexp using hex or octal
escapes. Probe for support of -P and prefer that over the original
approach.

Fixes: 4d037425dccf ("x86: Build check for embedded endbr64 instructions")
Signed-off-by: Jan Beulich <jbeulich@suse.com>
Acked-by: Andrew Cooper <andrew.cooper3@citrix.com>