Take "extern int maxbcachebuf" out of sys/param.h, since it breaks the
arm build.

In the arm build, elf_note.S includes sys/param.h and then does an
elf macro called ELFNOTE(). Although the compile error doesn't make
sense to me, I believe it just means that an "extern ..." can't exist
in param.h for this inclusion case.
I suspect adding #if !defined(LOCORE) might fix the build, but this
commit just takes the definition out.
I will ask freebsd-current@ what is the best was to deal with this
and do a subsequent commit after that.

Reported by: melounmichal@gmail.com