Files in directory ffcall/callback/trampoline_r of check-in c83183fa2721ebc5

This directory contains a reentrant version of the trampoline package.

Instead of clobbering a global variable, a pointer to data0,data1,...
is passed to the called function in a special CPU register.

The number of supported data words (data0,data1) is arbitrary; 3 or 4
or more would work as well if trampoline.c was modified appropriately.
Two words are needed, however, for passing closures through vacall_r
without an additional malloc() call per closure.

The include file <trampoline.h> is renamed to <trampoline_r.h>.