Add ".s" to source extensions.

It uses ".s" as the source for assembler on some toolchains including GCC.

tasks/mrbgem_spec.rake

@@ -58,13 +58,13 @@ module MRuby
         @linker = LinkerConfig.new([], [], [], [])
 
         @rbfiles = Dir.glob("#{dir}/mrblib/*.rb").sort
-        @objs = Dir.glob("#{dir}/src/*.{c,cpp,cxx,cc,m,asm,S}").map do |f|
+        @objs = Dir.glob("#{dir}/src/*.{c,cpp,cxx,cc,m,asm,s,S}").map do |f|
           objfile(f.relative_path_from(@dir).to_s.pathmap("#{build_dir}/%X"))
         end
         @objs << objfile("#{build_dir}/gem_init")
 
         @test_rbfiles = Dir.glob("#{dir}/test/*.rb")
-        @test_objs = Dir.glob("#{dir}/test/*.{c,cpp,cxx,cc,m,asm,S}").map do |f|
+        @test_objs = Dir.glob("#{dir}/test/*.{c,cpp,cxx,cc,m,asm,s,S}").map do |f|
           objfile(f.relative_path_from(dir).to_s.pathmap("#{build_dir}/%X"))
         end
         @test_preload = nil # 'test/assert.rb'