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

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

  $ make ARCH=um clean
  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
LDFLAGS_pcap.o or LDFLAGS_vde.o when cleaning.

While I was here, I removed the redundant -r option because it
already exists in the recipe.

Fixes: a83e4ca2 ("kbuild: remove cc-option switch from -Wframe-larger-than=")
Signed-off-by: Masahiro Yamada <masahiroy@kernel.org>
Reviewed-by: Nathan Chancellor <natechancellor@gmail.com>
Tested-by: Nathan Chancellor <natechancellor@gmail.com> [build]