gcc: disable libsanitizer for musl

A build issue affects libsanitizer on musl toolchains, even with
previous versions of gcc such as 4.8.x, so we disable building
libsanitizer when working with musl.

Signed-off-by: Thomas Petazzoni <thomas.petazzoni@free-electrons.com>
Signed-off-by: Peter Korsgaard <peter@korsgaard.com>