elfutils: towards uClibc support: solve memcpy problem

elfutils uses some strange internal alias of memcpy in glibc, so
workaround this when building with uClibc.

Heavily based from work done by Stefan Fröberg, but with many further
modifications by Thomas Petazzoni.

Signed-off-by: Thomas Petazzoni <thomas.petazzoni@free-electrons.com>
Signed-off-by: Peter Korsgaard <jacmet@sunsite.dk>