adding extern maxbcachebuf to param.h

[Rick Macklem]

My recent commit (r320062) broke the arm build when it added
extern int maxbcachebuf;
to sys/param.h. Although I don't understand the actual failure, I believe
it is caused by arm/arm/elf_note.S including param.h and then using the
ELFNOTE() macro.

As a temporary fix, I have committed r320070, which removes the definition
from sys/param.h.
This brings me to the question of how best to fix this?
1 - Just leave it the way it is now, where "extern int maxbcachebuf" isn't defined
     in a generic include file and needs to be defined as above before use.
2 - Add "!defined(LOCORE)" to the definition of it in sys/param.h, which I believe
     will also fix the problem.
3 - Put it in some other sys/*.h file which never gets included in assembler files.
     What .h would be appropriate?

Once I have answers to the above, I can update the fix.
Thanks, rick