Add -O2 to flang flags when building on WoA in Release mode

[martin-frbg]

fixes #5120

[rgommers]

LGTM. I can't test locally, but should work. I considered whether -O2 should work also with clang-cl (rather than /O2 style flag), and https://flang.llvm.org/docs/FlangCommandLineReference.html#optimization-level says it's always -O2.

[martin-frbg]

LGTM. I can't test locally, but should work. I considered whether -O2 should work also with clang-cl (rather than /O2 style flag), and https://flang.llvm.org/docs/FlangCommandLineReference.html#optimization-level says it's always -O2.

Thanks - I do have the hardware to test, and in fact it was one of the Qualcomm/WoA-associated engineers who raised the issue (mcw=Multicoreware Inc.)