package/ruby: links with atomic if needed

Build fails since bump to version 3.0.0 in commit
af5226f2fd1292a26f2dfda32f41cbbad7aa4cc because ruby needs atomic
operation support since
https://github.com/ruby/ruby/commit/6ed6b85ece8733518a7da0c3ec714f20d1102bf5

Fixes:
 - http://autobuild.buildroot.org/results/84ee5f4688be994a5440c3a61bddabee72ca3b3c

Signed-off-by: Fabrice Fontaine <fontaine.fabrice@gmail.com>
Signed-off-by: Peter Korsgaard <peter@korsgaard.com>

package/ruby/ruby.mk

@@ -23,6 +23,10 @@ RUBY_CPE_ID_VENDOR = ruby-lang
 # 0001-fix-default-coroutine-selection.patch
 RUBY_AUTORECONF = YES
 
+ifeq ($(BR2_TOOLCHAIN_HAS_LIBATOMIC),y)
+RUBY_CONF_ENV += LIBS=-latomic
+endif
+
 ifeq ($(BR2_TOOLCHAIN_USES_UCLIBC),y)
 # On uClibc, finite, isinf and isnan are not directly implemented as
 # functions.  Instead math.h #define's these to __finite, __isinf and