package/qemu: disable Qemu user-land emulation for musl toolchains

package/qemu: disable Qemu user-land emulation for musl toolchains

Like for cyclictest (rt-test)[1], linux-user/syscall.c use the
definition of the "struct sigevent" from Glibc/uClibc which doesn't
build with musl libc.

So, disable Qemu user-land emulation for musl toolchains.

Fixes:
http://autobuild.buildroot.net/results/b27/b277e668d9e22b624416bf46f6ccba802acd257e

[1] 43bffa59e7359610c8f00038ddd97795c0ca35e1

Signed-off-by: Romain Naour <romain.naour@gmail.com>
Cc: Francois Perrad <francois.perrad@gadz.org>
Signed-off-by: Thomas Petazzoni <thomas.petazzoni@free-electrons.com>