Use external stack if `stack` section wasn't specified

[cesarus777]

Reproduced on 1.0

I want just to generate a simple snippet and link it with a simple wrapper. To do this I created config file:

options:
  march: riscv64
  model-plugin: None
  seed: 0
  honor-target-abi: on

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

histogram:
  - [ADD, 1]

and called snippy:

llvm-snippy layout.yaml

Output:

error: LLVM ERROR: Cannot spill requested registers: no stack space allocated.

Expected behavior: use external stack if no stack section were specified.