buildsys: Fix setting LIBGCC

We were ending up with multiple entries in LIBGCC for recursive child
make instances, avoid that.

Signed-off-by: Bernhard Reutner-Fischer <rep.dot.nop@gmail.com>

Rules.mak

@@ -807,12 +807,12 @@ endif
 ASFLAGS += $(ASFLAG_--noexecstack)
 
 LIBGCC_CFLAGS ?= $(CFLAGS) $(CPU_CFLAGS-y)
-$(eval $(call cache-output-var,LIBGCC,$(CC) $(LIBGCC_CFLAGS) -print-libgcc-file-name))
+$(eval $(call cache-output-var,LIBGCC_A,$(CC) $(LIBGCC_CFLAGS) -print-libgcc-file-name))
 $(eval $(call cache-output-var,LIBGCC_EH,$(CC) $(LIBGCC_CFLAGS) -print-file-name=libgcc_eh.a))
 # with -O0 we (e.g. lockf) might end up with references to
 # _Unwind_Resume, so pull in gcc_eh in this case..
-LIBGCC_DIR:=$(dir $(LIBGCC))
-LIBGCC += $(if $(DODEBUG),$(LIBGCC_EH))
+LIBGCC_DIR := $(dir $(LIBGCC_A))
+LIBGCC := $(LIBGCC_A) $(if $(DODEBUG),$(LIBGCC_EH))
 
 # moved from libpthread/linuxthreads
 ifeq ($(UCLIBC_CTOR_DTOR),y)