ieee754 implementation of tan/atan

[Steve Munroe]

The current (2.2.4) implementation of tan/atan (./sysdeps/dbl-64/[s_tan.c |
s_atan.c) only export the symbols "tan" and "atan" while other "trig"
functions (sin, cos, ...) export "__" prefixed symbols (__sin, __cos, ...)
then export a weak alias for the unprefixed symbol (sin, cos, ...). Also
the float forms (sinf, cosf, tanf, atanf, asinf, acosf, ...) seem to follow
the form that exports "__" prefixed form with a weak alias for the
unprefixed form.

So at minimum the double tan/atan implementations are inconsistent with the
rest of the ieee754 implementation Also several architectures (68K and
i386) override the tan/atan implementations and do export both symbols
(tan/__tan, atan/__atan). Implementation like powerpc (32- and 64-bit),
which do not override the ieee754 implementation, are missing the
__tan/_atan symbols.

This seems like a bug in the ieee754 implementation for tan/atan ...