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.)

libm: Fix log-related symbol aliases.

- Do the weak alias dance for log, log2, log10, log1p, and their
f/l variants.

(Not strictly necessary for log2 and log10 since nothing in the
math library itself uses them -- only a computer scientist would
use log2 and only an engineer would use log10 -- but let's just do
it the same for all our forestry.)

- Provide long double aliases on vax.

- Sprinkle RCS ids.

branches: 1.1.2;
libm: Factor out common expected symbol list.

Should substantially reduce the maintenance burden.