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

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

Submitted by: Daniel Engberg (daniel.engberg.lists@pyret.net)
Reviewed by: emaste
Differential Revision: https://reviews.freebsd.org/D21021