unicore32: do not evaluate compiler's library path when cleaning

Since commit a83e4ca2 ("kbuild: remove cc-option switch from
-Wframe-larger-than="), 'make ARCH=unicore32 clean' emits error
messages as follows:

  $ make ARCH=unicore32 clean
  gcc: error: missing argument to '-Wframe-larger-than='
  gcc: error: missing argument to '-Wframe-larger-than='

We do not care compiler flags when cleaning.

Use the '=' operator for lazy expansion because we do not use
GNU_LIBC_A or GNU_LIBGCC_A when cleaning.

Fixes: a83e4ca2 ("kbuild: remove cc-option switch from -Wframe-larger-than=")
Signed-off-by: Masahiro Yamada <masahiroy@kernel.org>
Reviewed-by: Nick Desaulniers <ndesaulniers@google.com>

arch/unicore32/lib/Makefile

@@ -10,12 +10,12 @@ lib-y	+= strncpy_from_user.o strnlen_user.o
 lib-y	+= clear_user.o copy_page.o
 lib-y	+= copy_from_user.o copy_to_user.o
 
-GNU_LIBC_A		:= $(shell $(CC) $(KBUILD_CFLAGS) -print-file-name=libc.a)
+GNU_LIBC_A		= $(shell $(CC) $(KBUILD_CFLAGS) -print-file-name=libc.a)
 GNU_LIBC_A_OBJS		:= memchr.o memcpy.o memmove.o memset.o
 GNU_LIBC_A_OBJS		+= strchr.o strrchr.o
 GNU_LIBC_A_OBJS		+= rawmemchr.o			# needed by strrchr.o
 
-GNU_LIBGCC_A		:= $(shell $(CC) $(KBUILD_CFLAGS) -print-file-name=libgcc.a)
+GNU_LIBGCC_A		= $(shell $(CC) $(KBUILD_CFLAGS) -print-file-name=libgcc.a)
 GNU_LIBGCC_A_OBJS	:= _ashldi3.o _ashrdi3.o _lshrdi3.o
 GNU_LIBGCC_A_OBJS	+= _divsi3.o _modsi3.o _ucmpdi2.o _umodsi3.o _udivsi3.o