[libc] remove `-O3` from build rules

[nickdesaulniers]

Forking the discussion from here (cc @jyknight ). I don't think we should be hard coding -O3 throughout our cmake build files. Not only is it tedious, but it's very likely we will have customers that would prefer a binary sized optimized build (-Os). Instead, I think the widely understood CMAKE_BUILD_TYPE should be used to key off whether we should be building with -O2 vs -Os. If we need -O3, I think it would be good to understand where+why.

[llvmbot]

@llvm/issue-subscribers-libc

Author: Nick Desaulniers (nickdesaulniers)

Forking the discussion from [here](https://github.com//pull/88054#discussion_r1795989138) (cc @jyknight ). I don't think we should be hard coding `-O3` throughout our cmake build files. Not only is it tedious, but it's very likely we will have customers that would prefer a binary sized optimized build (`-Os`). Instead, I think the widely understood `CMAKE_BUILD_TYPE` should be used to key off whether we should be building with `-O2` vs `-Os`. If we need `-O3`, I think it would be good to understand where+why.

[lntue]

It was mostly for math functions. Maybe we can run some diff / perf to see how much different between -O2 and -O3 for math functions, and refactor it to / reuse a common flag that can be overwritten?