Make dbl-64 atan and tan into weak aliases.

Make dbl-64 atan and tan into weak aliases.

This patch converts the dbl-64 implementations of atan and tan into
weak aliases of __atan and __tan, in preparation for making them use
libm_alias_double.  Consequent changes are made to the x86_64
multiarch versions wrapping round them (with the dbl-64 functions,
like other such functions, being made not to define their aliases at
all if __atan or __tan are defined as macros by an including file).

Tested for x86_64, and with build-many-glibcs.py.

	* sysdeps/ieee754/dbl-64/s_atan.c (atan): Rename to __atan and
	define as weak alias of __atan.  Do not define any aliases if
	[__atan].
	[NO_LONG_DOUBLE] (__atanl): Define as strong alias of __atan.
	[NO_LONG_DOUBLE] (atanl): Define as weak alias of __atanl.
	* sysdeps/ieee754/dbl-64/s_tan.c (tan): Rename to __tan and define
	as weak alias of __tan.  Do not define any aliases if [__tan].
	[NO_LONG_DOUBLE] (__tanl): Define as strong alias of __tan.
	[NO_LONG_DOUBLE] (tanl): Define as weak alias of __tanl.
	* sysdeps/x86_64/fpu/multiarch/s_atan-avx.c (atan): Rename to
	__atan.
	* sysdeps/x86_64/fpu/multiarch/s_atan-fma.c (atan): Likewise.
	* sysdeps/x86_64/fpu/multiarch/s_atan-fma4.c (atan): Likewise.
	* sysdeps/x86_64/fpu/multiarch/s_atan.c (atan): Rename to __atan
	and define as weak alias of __atan.
	* sysdeps/x86_64/fpu/multiarch/s_tan-avx.c (tan): Rename to
	__atan.
	* sysdeps/x86_64/fpu/multiarch/s_tan-fma.c (tan): Likewise.
	* sysdeps/x86_64/fpu/multiarch/s_tan-fma4.c (tan): Likewise.
	* sysdeps/x86_64/fpu/multiarch/s_tan.c (tan): Rename to __tan and
	define as weak alias of __tan.