Unify Unix arm compiler optimization options (dotnet/coreclr#26299)

Unify Unix arm compiler optimization options (dotnet/coreclr#26299)

Long time ago, we were using clang 3.8 for building for Unix arm. There
was a codegen bug that forced us to set optimization for release build
to -O1 instead of -O3.
Now the minimum clang version we use for arm is 5.0 and I've verified
that binaries compiled with -O3 pass all the Coreclr pri 1 tests and so
the issue is gone.
So this change unifies the optimization levels for all architectures.

Commit migrated from https://github.com/dotnet/coreclr/commit/a7b43df9208036ebeb56bf0cd6c498aef3692407