libm: Factor out common expected symbol list.

Should substantially reduce the maintenance burden.

libm: Do the weak alias dance for asin, acos, atan.

These are used internally by the complex trig functions, so they need
weak aliases.

libm: Split expected symbols on arm and mips by hard/soft float.

libm: Do the weak alias dance for sin/cos/tan/finite{,f}.

tan isn't used internally, but it's confusing for it to be treated
differently from sin and cos, and there's no harm in doing the dance
unnecessarily.

libm: Memorialize expected symbols on various architectures.

This will reduce the risk of accidentally adding or deleting the
wrong symbols while fixing the aliases.

(This is all the architectures I have a build tree for handy; can add
other architectures like m68k later.)