[libc] Enable __llvm_libc_syscall and fork

This patch enables the remaining calls from unistd.

The test cases had to be updated to:
1. Use SYS_symlinkat if SYS_symlink is not available
2. Use SYS_readlinkat if SYS_readlink is not available
3. Use SYS_unlinkat if SYS_unlink is not available
4. Use SYS_openat if SYS_open is not available

We also abort compilation if neither of the syscalls mentioned above are
available.

Differential Revision: https://reviews.llvm.org/D146161