Summary:
Started failing in https://lab.llvm.org/buildbot/#/builders/73/builds/7977.

Summary:
This patch implements the `printf` family of functions on the GPU using
the new variadic support. This patch adapts the old handling in the
`rpc_fprintf` placeholder, but adds an extra RPC call to get the size of
the buffer to copy. This prevents the GPU from needing to parse the
string. While it's theoretically possible for the pass to know the size
of the struct, it's prohibitively difficult to do while maintaining ABI
compatibility with NVIDIA's varargs.

Depends on https://github.com/llvm/llvm-project/pull/96015.

Summary:
This patch adds a temporary implementation that uses a struct-based
interface in lieu of varargs support. Once varargs support exists we
will move this implementation to the "real" printf implementation.

Conceptually, this patch has the client copy over its format string and
arguments to the server. The server will then scan the format string
searching for any specifiers that are actually a string. If it is a
string then we will send the pointer back to the server to tell it to
copy it back. This copied value will then replace the pointer when the
final formatting is done.

This will require a built-in extension to the varargs support to get
access to the underlying struct. The varargs used on the GPU will simply
be a struct wrapped in a varargs ABI.

This is step 4 of
https://discourse.llvm.org/t/rfc-customizable-namespace-to-allow-testing-the-libc-when-the-system-libc-is-also-llvms-libc/73079

This generalizes handling of the integration tests. We now implicitly
depend on the `libc.startup.${LIBC_TARGET_OS}.crt1` file rather than
passing it in manually. This simplifies the interface.

Reviewed By: sivachandra

Differential Revision: https://reviews.llvm.org/D146237

This part of the effort to make all test related pieces into the `test`
directory. This helps is excluding test related pieces in a straight
forward manner if LLVM_INCLUDE_TESTS is OFF. Future patches will also move
the MPFR wrapper and testutils into the 'test' directory.

Reviewed By: lntue

Differential Revision: https://reviews.llvm.org/D140049

To accurately measure the size of sprintf in a finished binary, the
easiest method is to simply build a binary with and without sprintf.
This patch adds an integration test that can be built with and without
sprintf, as well as targets to build it.

Reviewed By: sivachandra

Differential Revision: https://reviews.llvm.org/D131735