How arbitrary is arbitrary? ARMv7's Thumb2 format immediates are composed of a 8-bit field shifted by up to 5 bits. So you can form any 32-bit variable, but with limited precision.
ARMv8 modified immediates can describe a contiguous run of ones followed by a contiguous run of zeros, and SWAR variations of the same. So you can describe things like a repeating 0x3f... for example.
Do either of those formats encompass the kinds of literals that you need in the V8 JIT?
> so you can use pc-relative addressing ... at a penalty of one word
Since the RISC-V PC-relative addressing capabilities are similar to ARMv8 (adrp) and x86-64 (rip-relative addressing), I would have though that this is basically a non-problem. You pay one more live register to hold the page address, but you also get more registers, so I would think it mostly washes out. Where do you pay a penalty?