arm: Use -O2 instead of -O as optimization flag

[diizzy]

Summary

When using Clang -O is equivalent to -O2, change it -O2 to make it consistent with other platforms.
Reference: https://clang.llvm.org/docs/ClangCommandLineReference.html#optimization-level

Test Plan

Tested on armv7, Orange Pi PC

[emaste]

Seems reasonable. Was originally introduced in rS173375 but Clang is the default compiler and we can assume that contemporary GCC won't have "the -O2 issues."