Remove __long_double_t.

sys/cdefs.h has a macro __long_double_t used in two places in glibc.
long double is a standard part of C since C89; there is no need for
such an alias for it.  This patch removes that macro and uses long
double directly everywhere.  As an implementation-namespace,
undocumented symbol, it should not be considered part of the API for
users, and codesearch.debian.net shows no sign of it being used
outside glibc in a way that would break with this patch.

Tested for x86_64.

	* misc/sys/cdefs.h (__long_double_t): Remove.
	* stdio-common/printf_fp.c (__printf_fp_l): Use long double
	instead of __long_double_t,
	* stdlib/strfmon_l.c (__vstrfmon_l): Likewise.