libroot: Ship empty libpthread.a and libm.a for POSIX and general compatibility.

A few weeks back, I spotted in the Musl FAQ that they apparently ship
empty libm.a and libpthread.a files (https://www.musl-libc.org/faq.html),
which they said was for POSIX compatibility. A bit of digging got me to
http://pubs.opengroup.org/onlinepubs/9699919799/utilities/c99.html which
says:

> It is unspecified whether the libraries libc.a, libl.a, libm.a, libpthread.a,
> librt.a, [OB] [Option Start] libtrace.a, [Option End] libxnet.a, or liby.a
> exist as regular files. The implementation may accept as -l option-arguments
> names of objects that do not exist as regular files.

So to follow the letter of the law, we only need to have the "c99" command
accept these; however, it appears all Linux and BSD cstdlibs accept them
no matter what compliance mode is in effect.

Discussed with PulkoMandy. This will make HaikuPorts' job a lot easier...