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: 1.5.2;
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: Don't export symbol `pio4' from s_cosl.c.

This appears to have been added unintentionally in the recent FreeBSD
long double update, and never went out in a release.

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.)