RISC-V: fix global symbol lookups for mpentry with lld

[mhorne]

Summary

This is a follow up to r356481. In locore.S, before virtual memory is
set up, we should avoid using indirect address lookups through the GOT.
Therefore we need to convert uses of the la instruction to lla, which
always generates an auipc/addi pair of instructions. This conversion was
done for the BSP case, but not the AP case, resulting in a fault
somewhere before mpva and a failure to bring APs online.

Reported by: lwhsu

Test Plan

Kernel build with clang + lld boots in qemu with -smp 2

[jrtc27]

Ah yes, good point, looks good to me.

[lwhsu]

Thanks for the patch and this works in my staging CI env.