ltrace: disable libunwind for uClibc toolchains

ltrace requires that libunwind is built with backtrace() support.
For the internal uClibc toolchain we don't enable it, and for external
uClibc toolchains we can't know.
It's also unavailable for static uClibc toolchains.
So just disable libunwind support for uClibc toolchains in general.
Fixes:
http://autobuild.buildroot.net/results/ee0/ee037a19590fb85c64f97f78f74bcfd4d7766706/



Signed-off-by: Gustavo Zacarias <gustavo@zacarias.com.ar>
Signed-off-by: Thomas Petazzoni <thomas.petazzoni@free-electrons.com>