aperf: fix compilation on x86-32 with -fPIC

ebx is used to store the GOT pointer when compiled with -fPIC, so it's
not usable by inline assembly.

https://bugs.gentoo.org/375967

Signed-off-by: Matt Turner <mattst88@gmail.com>
Signed-off-by: Dominik Brodowski <linux@dominikbrodowski.net>