Provide `tan` in libc

[petrhosek]

We're trying to build Pico SDK against LLVM libc and tan is one of the missing functions.

[llvmbot]

@llvm/issue-subscribers-libc

Author: Petr Hosek (petrhosek)

We're trying to build Pico SDK against LLVM libc and `tan` is one of the missing functions.