Gas: making md_long_jump_size mutable

[Hans-Peter Nilsson <hans-peter dot nilsson at axis dot com>]

Sensitive people might want to skip this message, as it only
matters to targets with "broken dot word" support.  (Though
there seems to be 13 of them, judging from number of definitions
of md_long_jump_size.)

I'd like to commit a CRIS variant called CRISv32.  Support for
crisv32-*-* is already in the imported config.sub and
config.guess.  The binutils support will be both as separate
crisv32-*-* targets and as options for the cris-*-* ELF targets.

However, CRISv32 needs a different value for md_long_jump_size
than CRIS.  I see two solutions: remove the "const" from the
extern declaration in symbols.c and write.c (!) or wrap them
#ifndef md_long_jump_size_declaration (or similar).  (Actually,
the first step would be to move declarations to tc.h but let's
keep that separate to the discussion.)

I'd prefer to just remove the "const".

(No, I'm not interested in alternative 3: remove BROKEN_DOT_WORD
support.  At least not for the time being. ;-)

brgds, H-P