package/libunwind: disable for x86 with uClibc toolchain

package/libunwind: disable for x86 with uClibc toolchain

libunwind use sigreturn() while building for x86 [1] but this function
is not available with uClibc-ng.

This throw a warning during libunwind build:
In file included from x86/Los-linux.c:4:0:
x86/Gos-linux.c: In function ‘_ULx86_local_resume’:
x86/Gos-linux.c:298:7: warning: implicit declaration of function ‘sigreturn’ [-Wimplicit-function-declaration]
       sigreturn (sc);
       ^

But any program trying to link against libunwind-generic.so fail to build:
[...]usr/lib/libunwind-generic.so: undefined reference to `sigreturn'
collect2: error: ld returned 1 exit status

Disable libunwind for x86 target when uClibc-ng is used.

Fixes:
http://autobuild.buildroot.net/results/54a/54afac8148cff5f3c17e83f80917fd9006948fe0//build-end.log

[1] http://git.savannah.gnu.org/gitweb/?p=libunwind.git;a=blob;f=src/x86/Gos-linux.c;h=17aebc2974af50eb0bf8292689b2ed22a4c97866;hb=HEAD#l299

Signed-off-by: Romain Naour <romain.naour@gmail.com>
Tested-by: "Yann E. MORIN" <yann.morin.1998@free.fr>
Acked-by: "Yann E. MORIN" <yann.morin.1998@free.fr>
Signed-off-by: Thomas Petazzoni <thomas.petazzoni@free-electrons.com>