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: Fix powerpc64 build.

- Include fenv.c and fma(3) symbols (which just use the FMADD
instruction).
- Note the .FN symbols in libm for the asm functions. The FN symbols
point at the function _descriptors_; the .FN symbols point at the
first instruction of the function.

XXX Unclear why we have the .FN symbols for asm functions but not for
C functions. I'm not sure we should be exporting them.

libm: Fix powerpc64 build.

- Include fenv.c and fma(3) symbols (which just use the FMADD
instruction).
- Note the .FN symbols in libm for the asm functions. The FN symbols
point at the function _descriptors_; the .FN symbols point at the
first instruction of the function.

XXX Unclear why we have the .FN symbols for asm functions but not for
C functions. I'm not sure we should be exporting them.