Disable loop vectorization for gfortran on ARM

[ACSimon33]

Description

Fixes #1160

Checklist

• The documentation has been updated.
• If the PR solves a specific issue, it is set to be closed on merge.

[langou]

Thanks. That's really good and helpful.

[TamarChristinaArm]

Hi, disabling vectorization is a really big hammer. could you perhaps for arm64|aarch64 instead of -fno-tree-vectorize use -march=armv8-a which will just disable the extra instructions which cause the problem. You could so do as well for arm if you don't support armv7-a and earlier.