Discard `.pdr` in the PSP linker script

`.pdr` holds MIPS procedure-descriptor debug data. It is not allocated, so
`--gc-sections` does not collect it, but the relocations kept for it by the
target's `--emit-relocs` still refer to the functions that were collected.
rust-lld reports each of those as "relocation refers to a discarded section";
a hello-world picks up 70 such warnings, now that cargo no longer suppresses
linker output.

Nothing on the PSP reads `.pdr`, so discard it alongside the other non-loadable
sections already listed. This also drops ~9% off the resulting EBOOT.

Fixes https://github.com/overdrivenpotato/rust-psp/issues/203 (in part)