Add nextpnr flag '--exit-on-failed-target-frequency'

[jthornblad]

Description

• Add flag so we can keep nextpnr standard behavior and if this flag is used we can get a fast exit if timing fails on placement.

• Update run_pnr.sh script to use this flag

Type of change

Please tick any that are relevant to this PR and remove any that aren't.

• Bugfix (non breaking change which resolve an issue)
• Feature (non breaking change which adds functionality)
• Breaking Change (a change which would cause existing functionality to not work as expected)
• Documentation (a change to documentation)

Submission checklist

• My code follows the style guidelines of this project
• I have performed a self-review of my changes
• I have tested and verified my changes on target
• My changes are well written and CI is passing
• I have squashed my work to relevant commits and rebased on main for linear history
• I have added a "Co-authored-by: x" if several people contributed, either pair programming or by squashing commits from different authors.
• I have updated the documentation where relevant (readme, dev.tillitis.se etc.)
• QEMU is updated to reflect changes

[dehanj]

Rebased on main. Works fine.
Now when our sed insert starts to be more complicated it starts to make more sense to have a fork. It might be easier to maintain.