PR/59304: kernel longjmp(9) fails to make setjmp(9) return 1

Fix riscv longjmp.

branches: 1.6.2;
risc-v: fix the error code when uvm_fault fails with cpu_set_onfault

Return the error from uvm_fault instead of EFAULT unconditionally when
faulting with cpu_set_onfault to fix several atf tests.

RISC-V support that works on QEMU with a single hart.

Thanks for Simon Burge for plic(4).

riscv: Optimization: Omit needless membar when triggering softint.

When we are triggering a softint, it can't already hold any mutexes.
So any path to mutex_exit(mtx) must go via mutex_enter(mtx), which is
always done with atomic r/m/w, and we need not issue any explicit
barrier between ci->ci_curlwp = softlwp and a potential load of
mtx->mtx_owner in mutex_exit.

PR kern/57240

riscv: Add missing barriers in cpu_switchto.

Details in comments.

PR kern/57240

branches: 1.2.2;
Restore t5 and t6 from the correct locations in exception_kernexit.

From Simon.

Split out a bunch of functions from locore.S into cpu_switch.S

NFC