Makefile: Convert -Wimplicit-fallthrough=3 to just -Wimplicit-fallthrough for clang

A compilation -Wimplicit-fallthrough warning was enabled by commit
a035d552a9 ("Makefile: Globally enable fall-through warning")

Even though clang 10.0.0 does not currently support this warning without
a patch, clang currently does not support a value for this option.

  Link: https://bugs.llvm.org/show_bug.cgi?id=39382

The gcc default for this warning is 3 so removing the =3 has no effect
for gcc and enables the warning for patched versions of clang.

Also remove the =3 from an existing use in a parisc Makefile:
arch/parisc/math-emu/Makefile

Signed-off-by: Joe Perches <joe@perches.com>
Reviewed-and-tested-by: Nathan Chancellor <natechancellor@gmail.com>
Cc: Gustavo A. R. Silva <gustavo@embeddedor.com>
Signed-off-by: Linus Torvalds <torvalds@linux-foundation.org>

arch/parisc/math-emu/Makefile

@@ -18,4 +18,4 @@ obj-y	 := frnd.o driver.o decode_exc.o fpudispatch.o denormal.o \
 # other very old or stripped-down PA-RISC CPUs -- not currently supported
 
 obj-$(CONFIG_MATH_EMULATION)	+= unimplemented-math-emulation.o
-CFLAGS_REMOVE_fpudispatch.o	= -Wimplicit-fallthrough=3
+CFLAGS_REMOVE_fpudispatch.o	= -Wimplicit-fallthrough