Fix compilation of locore.S with clang

[mhorne]

Summary

The branch from _start to mpentry has to cross a large section of data;
an offset larger than can be specified with a 12-bit branch immediate.
Fix this by converting the branch to an unconditional jump. The gcc
assembler does this conversion silently but is not done automatically by
clang.

Reported by: Jeremy Bennett <[e-mail]>

[markj]

sys/riscv/riscv/locore.S

73

I would suggest adding a comment, since otherwise it looks like something that should be simplified.

[mhorne]

Add comment explaining the change

[markj]

Approved.