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>

package/libunwind/Config.in

@@ -15,7 +15,7 @@ config BR2_PACKAGE_LIBUNWIND_ARCH_SUPPORTS
 		 BR2_mips64el || BR2_powerpc || BR2_sh || BR2_i386 || BR2_x86_64)
 	default y if BR2_TOOLCHAIN_USES_UCLIBC && \
 		(((BR2_arm || BR2_armeb) && BR2_ARM_CPU_HAS_ARM) || \
-		 BR2_i386 || BR2_mips || BR2_mipsel || \
+		 BR2_mips || BR2_mipsel || \
 		 BR2_mips64 || BR2_mips64el || BR2_x86_64)
 	default y if BR2_TOOLCHAIN_USES_MUSL && \
 		(((BR2_arm || BR2_armeb) && BR2_ARM_CPU_HAS_ARM) || \