asinl: Simplify for platforms where 'long double' == 'double'. * lib/asinl.c (asinl) [HAVE_SAME_LONG_DOUBLE_AS_DOUBLE]: New alternative implementation. * m4/asinl.m4 (gl_FUNC_ASINL): Require gl_LONG_DOUBLE_VS_DOUBLE. Determine ASINL_LIBM according to HAVE_SAME_LONG_DOUBLE_AS_DOUBLE. * modules/asinl (Depends-on): Add asin. Update conditions.