[libc] Implement atan2l

[petrhosek]

Some of our embedded projects use libc++ complex header which uses atan2l:

llvm-project/libcxx/include/complex

Line 953 in 800593a

return std::atan2l(0.L, __re);

[llvmbot]

@llvm/issue-subscribers-libc

Author: Petr Hosek (petrhosek)

Some of our embedded projects use libc++ `complex` header which uses `atan2l`: https://github.com/llvm/llvm-project/blob/800593a014791e3aa26f04357f580d2dcc87320a/libcxx/include/complex#L953