libm: Fix powerpc64 expected symbols.

We no longer list the linker-generated ._init/._fini.

PR lib/58838: shared libraries in base should all have expsym lists

bsd.lib.mk: Exclude MD linker-generated crud from expected symbols.

Not entirely sure why these symbols like _init/_fini/_end on x86 or
__bss_start__/__bss_end__ on aarch64 or _GLOBAL_OFFSET_TABLE_ on hppa
are exported at all, but it's something the linker is doing
automatically -- and unnecessarily, since libraries with explicit
version scripts don't export these -- and I'm not sure there's any way
to suppress it.

This way, most *.expsym lists can be machine-independent, which will
help keep maintenance burden much lower.

Currently we have only one *.expsym file for a library that _doesn't_
use a version script, libm -- fix that up to reduce the crud. (That
one already happens to have a lot of machine-dependent exports
because of differences in floating-point ABIs so the linker-generated
MD symbol crud wasn't much more of a maintenance burden in this case,
but this maintenance burden is why I haven't aggressively gone around
creating *.expsym files for other libraries.)

branches: 1.3.2;
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.