Fix strfmon_l: Use specified locale for number formatting [BZ #19633]

The commit 985fc132
"strfmon_l: Use specified locale for number formatting [BZ #19633]"
introduced an elf/check-abi-libc testfailure due to __printf_fp_l
on architectures which use sysdeps/ieee754/ldbl-opt/math_ldbl_opt.h.

This patch uses libc_hidden_def instead of ldbl_hidden_def.
The ldbl_strong_alias is removed due to the rename of ___printf_fp_l
to __printf_fp_l.

ChangeLog:

	* stdio-common/printf_fp.c (__printf_fp_l):
	Rename ___printf_fp_l to __printf_fp_l and
	remove strong alias. Use libc_hidden_def instead
	of ldbl_hidden_def macro.

(cherry picked from commit b1b8f5d8)
(cherry picked from commit a37d61a6)