Fix clean in isa/ with non-default compiler (riscv-software-src#443)

The list of active tests in isa/ depends, via the COMPILER_SUPPORTS_*
macros, on running the RISC-V compiler, so pass the necessary options
when invoking make clean there.

Makefile.in

@@ -37,7 +37,7 @@ debug-check-fast:
 	$(MAKE) -C debug -f $(debug_src_dir)/Makefile src_dir=$(debug_src_dir) XLEN=$(XLEN) spike$(XLEN)
 
 clean:
-	[ ! -d isa ]        || $(MAKE) -C isa -f $(isa_src_dir)/Makefile src_dir=$(isa_src_dir) clean
+	[ ! -d isa ]        || $(MAKE) -C isa -f $(isa_src_dir)/Makefile src_dir=$(isa_src_dir) XLEN=$(XLEN) $(RISCV_PREFIX_VAR) clean
 	[ ! -d benchmarks ] || $(MAKE) -C benchmarks -f $(bmarkdir)/Makefile src_dir=$(bmarkdir) clean
 	[ ! -d debug ]      || $(MAKE) -C debug -f $(debug_src_dir)/Makefile src_dir=$(debug_src_dir) clean