Add a callback from gcc to determine wether the bridge is available or
not.
gcc/rust/ChangeLog:
* expand/rust-proc-macro.cc (available): Add symbol
registration.
(load_macros_array): Likewise.
libgrust/ChangeLog:
* libproc_macro/proc_macro.cc (not_available): Add a
function to express bridge unavailability.
* libproc_macro/proc_macro.h (not_available): Likewise.
* libproc_macro/registration.h: Add symbol type.
Signed-off-by: Pierre-Emmanuel Patry <pierre-emmanuel.patry@embecosm.com>
c0763eac05