refactor: change definition of time_t in TEEOS

[dybucc]

Description

This PR aims to both discuss and (possibly) change the bit-width of the time_t type in TEEOS. At
present, we expose a single time_t type whose bit-width relies on semantics specific to each
target system, as c_long is likely to be 64-bit wide under platforms following LP64 and 32-bit
wide under platforms following LLP64, but there's a few things that lead me to believe this may
cause conflicts in TEEOS.

The first issue I would like to address is that if we're exposing the time_t definition in the
OhosOS SDK that TEEOS uses, our current definition is not completely correct. SDKs for all three of
Darwin, Linux and Windows use the same definition as musl; Namely, typedef _Int64 time_t. In
libc, though, we expose it as a c_long.

Just to be sure, though, I then looked at the TEEOS upstream kernel repo, and they define time_t
in terms of either one of a 32-bit unsigned integer or a 64-bit unsigned integer, depending on the
machine's word size. That implies this target OS likely supports running on both such types of
machines, which means that our current definition is sort of OK on the bitwidth side but not on the
signed integer side.

The proposed change in this PR switches the type to be an explicit 64-bit signed integer, though
this has not taken into account the definition in the TEEOS kernel. That's why I would like to spark
some discussion on this, or otherwise call it a day and learn something new.

Sources

• A regex search through the downloadable OhosOS SDK shows that all type definitions accross
  architectures follow musl's. For reference, I ran ripgrep across the SDKs for Darwin, Linux and
  Windows with the following command:

  rg -g "**/alltypes.h" -e "\btime_t\b"

  I also ran it by more generally scoping the search to header files, but relevant results were the
  same.

• TEEOS kernel sources defining time_t as an unsigned integer.

  https://github.com/openharmony/tee_tee_os_kernel/blob/fc85968fa01f6503997a50f09f1fc11e832860d9/kernel/include/common/types.h#L50

Checklist

• Relevant tests in libc-test/semver have been updated
• No placeholder or unstable values like *LAST or *MAX are
  included (see #3131)
• Tested locally (cd libc-test && cargo test --target mytarget);
  especially relevant for platforms that may not be checked in CI

[dybucc]

CI seems to have failed for reasons unrelated to the changes introduced in this PR. Rerunning it
will likely solve it.

[rustbot]

This PR was rebased onto a different main commit. Here's a range-diff highlighting what actually changed.

Rebasing is a normal part of keeping PRs up to date, so no action is needed—this note is just to help reviewers.