sysconf(_SC_PAGESIZE) is implemented now.

Reviewers: michaelrj-google, aaronmondal, rupprecht, keith

Reviewed By: michaelrj-google

Pull Request: https://github.com/llvm/llvm-project/pull/163462

Previously they called sysconf and capget. Those were blocking adding
bazel rules for those tests since sysconf isn't done.

Remove functions depending on each other, cleanup internal errno in
shm_common, remove various unnecessary includes.

This is the first step in preparation for:
https://discourse.llvm.org/t/rfc-make-clang-builtin-math-functions-constexpr-with-llvm-libc-to-support-c-23-constexpr-math-functions/86450

See PR #132823 for a previous change to sys/epoll tests, and prior
changes before that.

ErrnoCheckingTest ensures that errno is properly reset at the beginning
of the test case, and is validated at the end of it. This change removes
many instances of manual errno manipulation.

Some tests were including LibcTest.h directly. Instead you should
include Test.h which does proper indirection for other test frameworks
we support (zxtest, gtest). Also added some license headers to tests
that were missing them.

We shouldn't be including headers directly from asm-generic for macros.
It's safer to get those through the correct primary header where
possible (e.g. fcntl instead of asm-generic/fcntl).

For our public headers we may need to include the asm-generic
headers instead of defining all the macros ourselves, but that's
something for a followup PR.

Having libc_errno outside of the namespace causes versioning issues when
trying to link the tests against LLVM-libc. Most of this patch is just
moving libc_errno inside the namespace in tests. This isn't necessary in
the function implementations since those are already inside the
namespace.

I suspect this is a bug in linux 4.19, as the test passes as written on
my
linux 6.5 machine.

Let's revisit this after the build bots are upgraded.

Link: #80073