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

A compilation -Wimplicit-fallthrough warning was enabled by commit
a035d552a93b ("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 <[e-mail]>
Reviewed-and-tested-by: Nathan Chancellor <[e-mail]>
Cc: Gustavo A. R. Silva <[e-mail]>
Signed-off-by: Linus Torvalds <[e-mail]>

Makefile

···
 KBUILD_CFLAGS += -Wdeclaration-after-statement
 
 # Warn about unmarked fall-throughs in switch statement.
-KBUILD_CFLAGS += $(call cc-option,-Wimplicit-fallthrough=3,)
+KBUILD_CFLAGS += $(call cc-option,-Wimplicit-fallthrough,)
 
 # Variable Length Arrays (VLAs) should not be used anywhere in the kernel
 KBUILD_CFLAGS += -Wvla

arch/parisc/math-emu/Makefile

···
 # 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