Link libXrdServer to libatomic on riscv64

[ellert]

Link libXrdServer to libatomic on riscv64 to avoid undefined references:

/usr/bin/ld: libXrdServer.so.3.0.0: undefined reference to `__atomic_fetch_or_1'
/usr/bin/ld: libXrdServer.so.3.0.0: undefined reference to `__atomic_fetch_sub_1'
/usr/bin/ld: libXrdServer.so.3.0.0: undefined reference to `__atomic_fetch_and_1'
/usr/bin/ld: libXrdServer.so.3.0.0: undefined reference to `__atomic_fetch_add_1'
collect2: error: ld returned 1 exit status

[ellert]

This didn't help - marking work in progress.

[ellert]

This solution works! Remove work in progress marker.