Revert "[GCC]: Fix fence.i bug (#296)"

This reverts commit 54de960a5e.

See #298 for explanation.

Makefile.in

@@ -57,27 +57,16 @@ endif
 # *** If --with-abi is given but --with-arch is not, then the ISA uses the
 #     compiler's default.
 
-
-ifneq (@WITH_ARCH@,)
-  is_32bit := $(findstring 32,@WITH_ARCH@)
-else
-ifneq (@host_alias@,)
-	is_32bit := $(findstring 32,@host_alias@)
-else
-	is_32bit := ""
-endif
-endif
-
 ifneq (@WITH_ARCH@,)
   march := -march=@WITH_ARCH@
-else
-	march := -march=$(if $(is_32bit),rv32,rv64)im_zicsr_zifencei
+  is_32bit := $(findstring 32,$(march))
+  ifeq (@WITH_ABI@,)
+    mabi := -mabi=$(if $(is_32bit),ilp32,lp64)
+  endif
 endif
 
 ifneq (@WITH_ABI@,)
   mabi := -mabi=@WITH_ABI@
-else
-  mabi := -mabi=$(if $(is_32bit),ilp32,lp64)
 endif
 
 # Installation directories