toolchain-external: install libatomic

It's required in some 32-bit architectures for the extended (64-bit)
atomic operations, like __sync_add_and_fetch_8.
These arches are at least: i386, mips & mipsel.

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

toolchain/toolchain-external/toolchain-external.mk

@@ -64,7 +64,7 @@
 #  of Buildroot is handled identical for the 2 toolchain types.
 
 ifeq ($(BR2_TOOLCHAIN_EXTERNAL_GLIBC)$(BR2_TOOLCHAIN_EXTERNAL_UCLIBC),y)
-LIB_EXTERNAL_LIBS += libc.so.* libcrypt.so.* libdl.so.* libgcc_s.so.* libm.so.* libnsl.so.* libresolv.so.* librt.so.* libutil.so.*
+LIB_EXTERNAL_LIBS += libatomic.so.* libc.so.* libcrypt.so.* libdl.so.* libgcc_s.so.* libm.so.* libnsl.so.* libresolv.so.* librt.so.* libutil.so.*
 ifeq ($(BR2_TOOLCHAIN_EXTERNAL_GLIBC)$(BR2_ARM_EABIHF),yy)
 LIB_EXTERNAL_LIBS += ld-linux-armhf.so.*
 else