apparently I get a misaligned load trap … in the misaligned_load_trap handler 
(so it’s different from the one described here Linux does not boot after a massive code addition to the bbl)
0000000080003870 <misaligned_load_trap>:
80003870: 000a07b7 lui a5,0xa0
80003874: 458d li a1,3
80003876: 3007a6f3 csrrs a3,mstatus,a5
8000387a: 00066703 lwu a4,0(a2)
8000387e: 30069073 csrw mstatus,a3
80003882: 00377813 andi a6,a4,3
80003886: 8eaa mv t4,a0
80003888: 1141 addi sp,sp,-16
8000388a: 8536 mv a0,a3
8000388c: 4f11 li t5,4
8000388e: 00b80363 beq a6,a1,80003894 <misaligned_load_trap+0x24>
80003892: 4f09 li t5,2
Edit: okay, this issue was funny. Apparently the compiler is generating RISCV-C but not defining __riscv_compressed, resulting in the trap handler getting unexpected input (the other __riscv_* are defined though?!? edit: this has been reported, is a known issue in gcc mainline [1]). After defining that manually and recompiling riscv-pk it works and boots !
medusa:~# uname -r
5.1.0-rc1-00010-ge3827dbacb90