The current implementation of `KINST_TRAMP_INIT` is working only on amd64,
where the breakpoint instruction is one byte long, which might not be the case
for other architectures (e.g in RISC-V it's either 2 or 4 bytes). This patch
introduces two machine-dependent constants, `KINST_TRAMP_FILL` and
`KINST_TRAMP_FILL_SIZE`, which hold the fill instruction (might not always be
`KINST_PATCHVAL`, as is the case in RISC-V) and the size of that instruction in
bytes respectively.