[libc][bazel] Simplify libc_build_rules by grouping release copts (#121630)

Extract all compiler options used to build "release" versions of libc
API functions into a separate helper function, instead of burying this
logic inside libc_function() macro.

With this change, we further split two "flavors" of cc_library()
produced for each libc public function:

* `<function>.__internal__` library used in unit tests is *not* built
with release copts and is thus indistinguishable from regular
libc_support_library(). Arguably, it's a good thing, because all sources
in a unit test are built with the same set of compiler flags, instead of
"franken-build" when a subset of sources is always built with -O3. If a
user needs to run the tests in optimized mode, they should really be
using Bazel invocation-level compile flags instead.
* `<function>` library that libc users can use to construct their own
static archive *is* built with the same release copts as before. There
is a pre-existing problem that its libc_support_library() dependencies
are not built with the same copts. We're not addressing it here now.