Various changes to the garbage collector and other relocators to

Various changes to the garbage collector and other relocators to
accomodate machines where jsr-style instructions do not encode the
target address directly and contiguously.

Some changes to cmp68kgc.h to better match the portable version,
cmpgc-portable.h .

Split the defaulting of various macros from gccode.h into
cmpgc-stub.h .  gccode.h includes cmpgc.h which should be a copy of (or
link to) cmp68kgc.h, cmpvaxgc.h, cmpgc-portable.g, or cmpgc-stub.h