Change Makefile rule for assembly to use .S rather than .s

It is more conventional to use compiler to pre-process and
assemble the `.S` file rather than forcing Makefile to use `.s`.

git-svn-id: svn+ssh://ci.ruby-lang.org/ruby/trunk@65952 b2dd03c8-39d4-4d8f-98ff-823fe69b080e

Makefile.in

@@ -213,7 +213,7 @@ DTRACE_REBUILD= @DTRACE_REBUILD@
 DTRACE_GLOMMED_OBJ = $(DTRACE_REBUILD:yes=ruby-glommed.$(OBJEXT))
 
 OBJEXT        = @OBJEXT@
-ASMEXT        = s
+ASMEXT        = S
 SOEXT         = @SOEXT@
 DLEXT         = @DLEXT@
 MANTYPE	      = @MANTYPE@