proc-macro-srv: make usage of RTLD_DEEPBIND portable

proc-macro-srv: make usage of RTLD_DEEPBIND portable

the constant is wrong on some platforms (e.g., on mips64el it's 0x10, and 0x8
is RTLD_NOLOAD which makes all this functionality broken), the libc crate takes
care of those differences for us.

fallback to old hard-coded value for non-glibc environments (which might or
might not of DEEPBIND support).

Forwarded: https://github.com/rust-lang/rust/pull/135591

Signed-off-by: Fabian Grünbichler <f.gruenbichler@proxmox.com>
Gbp-Pq: Topic behaviour
Gbp-Pq: Name proc-macro-srv-make-usage-of-RTLD_DEEPBIND-portable.patch