Fix -Os getc_unlocked linknamespace, localplt issues (bug 15105, bug 19463).

Fix -Os getc_unlocked linknamespace, localplt issues (bug 15105, bug 19463).

Continuing the fixes for linknamespace and localplt test failures with
-Os that arise from functions not being inlined in that case, this
patch fixes such failures for getc_unlocked.

__getc_unlocked already exists; this patch makes it explicitly hidden,
calls it where needed for namespace reasons, adds an inline function
for it when inline functions are used and adds libc_hidden_proto /
libc_hidden_weak for getc_unlocked.

Tested for x86_64 (both without -Os to make sure that case continues
to work, and with -Os to make sure all the relevant linknamespace and
localplt test failures are resolved).  Because of other such failures
that remain after this patch, neither of the bugs can yet be closed.

	[BZ #15105]
	[BZ #19463]
	* libio/getc_u.c (getc_unlocked): Use libc_hidden_weak.
	* include/stdio.h [!_ISOMAC] (__getc_unlocked): Use
	attribute_hidden, and define inline if [__USE_EXTERN_INLINES].
	[!_ISOMAC] (getc_unlocked): Use libc_hidden_proto.
	* misc/getttyent.c (__getttyent): Call __getc_unlocked instead of
	getc_unlocked.
	* time/tzfile.c (__tzfile_read): Likewise.