model-plugin option is not unset by default

[cesarus777]

Reproduced on 1.0

There is no model that is provided with snippy but generation with the simplest layout would fail due to fail to find libRISCVModel.so.

layout.yaml:

options:
  march: riscv64

sections:
  - name: text
    VMA: 0x1000
    LMA: 0x1000
    SIZE: 0x1000
    ACCESS: rx
  - name: data
    VMA: 0x2000
    LMA: 0x2000
    SIZE: 0x1000
    ACCESS: rw

histogram:
  - [ADD, 1]

Command:

llvm-snippy layout.yaml

Output:

warning: no instructions seed specified, using auto-generated one: 1713058880954237867
ld.lld: warning: cannot find entry symbol _start; not setting start address
LLVM ERROR: could not find library for plugin: libRISCVModel.so

Can model plugin be turned off by default since no default model is supplied with llvm-snippy?