asm-prototypes: Clear any CPP defines before declaring the functions

The asm-prototypes.h file is used to provide dummy function declarations
for genksyms, when processing asm files with EXPORT_SYMBOL. Make sure
that any architecture defines get out of our way. x86 currently has an
issue with memcpy on 64bit with CONFIG_KMEMCHECK=y and with
memset/__memset on 32bit:

	$ cat init/test.c
	#include <asm/asm-prototypes.h>
	$ make -s init/test.o
	In file included from ./arch/x86/include/asm/string.h:4:0,
			 from ./include/linux/string.h:18,
			 from ./include/linux/bitmap.h:8,
			 from ./include/linux/cpumask.h:11,
			 from ./arch/x86/include/asm/cpumask.h:4,
			 from ./arch/x86/include/asm/msr.h:10,
			 from ./arch/x86/include/asm/processor.h:20,
			 from ./arch/x86/include/asm/cpufeature.h:4,
			 from ./arch/x86/include/asm/thread_info.h:52,
			 from ./include/linux/thread_info.h:25,
			 from ./arch/x86/include/asm/preempt.h:6,
			 from ./include/linux/preempt.h:59,
			 from ./include/linux/spinlock.h:50,
			 from ./include/linux/seqlock.h:35,
			 from ./include/linux/time.h:5,
			 from ./include/uapi/linux/timex.h:56,
			 from ./include/linux/timex.h:56,
			 from ./include/linux/sched.h:19,
			 from ./include/linux/uaccess.h:4,
			 from ./arch/x86/include/asm/asm-prototypes.h:2,
			 from init/test.c:1:
	./arch/x86/include/asm/string_64.h:52:47: error: expected declaration specifiers or ‘...’ before ‘(’ token
	 #define memcpy(dst, src, len) __inline_memcpy((dst), (src), (len))
	 ./include/asm-generic/asm-prototypes.h:6:14: note: in expansion of macro ‘memcpy’
	  extern void *memcpy(void *, const void *, __kernel_size_t);

						       ^
	...

During real build, this manifests itself by genksyms segfaulting.

Fixes: 334bb773 ("x86/kbuild: enable modversions for symbols exported from asm")
Reported-and-tested-by: Borislav Petkov <bp@alien8.de>
Cc: Adam Borowski <kilobyte@angband.pl>
Signed-off-by: Michal Marek <mmarek@suse.com>