IMPORT: ebtree: only use __builtin_prefetch() when supported

IMPORT: ebtree: only use __builtin_prefetch() when supported

It looks like __builtin_prefetch() appeared in gcc-3.1 as there's no
mention of it in 3.0's doc. Let's replace it with eb_prefetch() which
maps to __builtin_prefetch() on supported compilers and falls back to
the usual do{}while(0) on other ones. It was tested to properly build
with tcc as well as gcc-2.95.

This is ebtree commit 7ee6ede56a57a046cb552ed31302b93ff1a21b1a.