nptl: mark symbols with libc forwarder hidden

nptl: mark symbols with libc forwarder hidden

Add attribute_hidden to all symbols having libc forwarder. This prevents
recursive self calls which would happen if libc is before libpthread in
linking order: the forwarder functions would call itself via the function
table, since the libpthread symbols would get overwritten with libc ones.
This has not been a problem in glibc since there these symbols are marked
hidden with linker version-script. Since we don't use one, we need to mark
these explicitly.

Signed-off-by: Timo Teräs <timo.teras@iki.fi>
Signed-off-by: Austin Foxley <austinf@cetoncorp.com>