Use libc_hidden_* for tolower, toupper (bug 15105).

Continuing the fixes for localplt test failures with -Os arising from
functions not being inlined in that case, this patch fixes such
failures for tolower and toupper by using libc_hidden_proto and
libc_hidden_def.

Tested for x86_64 (both that it removes this particular localplt
failure for -Os, and that the testsuite continues to pass without
-Os).

2018-02-22  Joseph Myers  <joseph@codesourcery.com>

	[BZ #15105]
	* ctype/ctype.c (tolower): Use libc_hidden_def.
	(toupper): Likewise.
	* include/ctype.h [!_ISOMAC] (tolower): Use libc_hidden_proto.
	[!_ISOMAC] (toupper): Likewise.

(cherry picked from commit 54412d20)