Change proc macro entrypoint from a fixed constant declaration to a
proper generation from the stable crate id. Although the stable crate id
is not in use yet, the mechanism to handle it is.
gcc/rust/ChangeLog:
* expand/rust-proc-macro.cc (CustomDeriveProcMacro::CustomDeriveProcMacro):
Remove constant string declaration.
(load_macros_array): Add call to the new generation function.
(generate_proc_macro_decls_symbol): Add a new function to generate the
entrypoint symbol name from the stable crate id.
(PROC_MACRO_DECLS_FMT_ARGS):
New macro to keep formats arguments in sync between each call.
* expand/rust-proc-macro.h (generate_proc_macro_decls_symbol): Add
function prototype.
* rust-system.h: Include <iomanip>.