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>