More backref performance - no strncmp

[Paolo Bonzini]

Actually, note that strncmp is not as extremely optimized as memcmp (the
loop is unrolled, but no strange tricks).  So the most highly optimized
function, in the context of regex matching, turns out to be the slowest
because of the startup overheads!  Since my_memcmp is as unoptimized as
strncmp, but is inlined and simpler, it makes sense that it is faster.

Paolo