Avoid compiler crash when building swgl optimised for ESR 91

[llewelld]

Building swgl with -O2 for aarch64 resulted in a segfault:

internal compiler error: Segmentation fault

Reducing optimisation to -O1 worked around this, see f64b6cf84dc6f6a6d35600d156e2c561e3f24293.

Ideally we would either:

1. Figure out why the compiler crash is actually happening and fix it; or
2. Target the optimisation more carefully, for example so it only applies to swgl.