Remove $(ASFLAGS) from build system and assemble with $(CFLAGS) instead

We already assemble our assembly files using the $(CC) compiler driver,
rather than the actual $(AS) assembler. This means that

* The C preprocessor gets run on the assembly file
* It's valid to pass gcc-style flags to it, like e.g.
  -mbranch-protection or -fcf-protection
* If you do so, the relevant preprocessor macros like __CET__ get set
* If you really wanted to pass assembler flags, you would need to do
  that using -Wa,... anyway

So I think it makes sense to pass "$(XCFLAGS) $(CFLAGS) $(CPPFLAGS)" to
gcc/clang/etc when assembling, rather than passing $(ASFLAGS) (since
the flags are not actually passed to `as`, but `cc`!).

The side effect of this is that if there are mitigation flags like
-fcf-protection in $CFLAGS, then the relevant macros like __CET__ will
be defined when assembling the files.

[Bug #20601]

configure.ac

@@ -489,7 +489,6 @@ AC_CACHE_CHECK([for $AR flags], [rb_cv_arflags], [
 	[rb_cv_arflags=rcD], [rb_cv_arflags=rcu])
 ])
 AC_SUBST(ARFLAGS, ["$rb_cv_arflags "])
-AC_SUBST(ASFLAGS)
 
 AS_CASE(["$target_os"],
 [cygwin*|msys*|mingw*], [

template/Makefile.in

@@ -225,7 +225,6 @@ AR            = @AR@
 ARFLAGS       = @ARFLAGS@$(empty)
 RANLIB        = @RANLIB@
 AS            = @AS@
-ASFLAGS       = @ASFLAGS@ $(ARCH_FLAG) $(INCFLAGS)
 IFCHANGE      = $(SHELL) $(tooldir)/ifchange
 OBJDUMP       = @OBJDUMP@
 OBJCOPY       = @OBJCOPY@
@@ -449,7 +448,7 @@ $(srcdir)/enc/jis/props.h: enc/jis/props.kwd
 
 .$(ASMEXT).$(OBJEXT):
 	@$(ECHO) assembling $<
-	$(Q) $(CC) $(ASFLAGS) -DSYMBOL_PREFIX=$(SYMBOL_PREFIX) -o $@ -c $<
+	$(Q) $(CC) $(CFLAGS) $(XCFLAGS) $(CPPFLAGS) $(COUTFLAG)$@ -DSYMBOL_PREFIX=$(SYMBOL_PREFIX) -c $<
 
 .c.$(ASMEXT):
 	@$(ECHO) translating $<