We have some modifications downstream to compile the flang runtime for
amdgpu using clang OpenMP, some more hacky than others to workaround
(hopefully temporary) compiler issues. The additions here are the
non-hacky alterations.

Main changes:
* Create freestanding versions of memcpy, strlen and memmove, and
replace std:: references with these so that we can default to std:: when
it's available, or our own Flang implementation when it's not. * Wrap
more bits and pieces of the library in declare target wrappers (RT_*
macros). * Fix some warnings that'll pose issues with werror on, in this
case having the namespace infront of variables passed to templates.

Another minor issues that'll likely still pop up depending on the
program you're linking with is that abort will be undefined, it is
perhaps possible to solve it with a freestanding implementation as with
memcpy etc. but we end up with multiple definitions in this case. An
alternative is to create an empty extern "c" version (which can be empty
or forwrd on to the builtin).

Co-author: Dan Palermo Dan.Palermo@amd.com

Implement extended intrinsic PUTENV, both function and subroutine forms.
Add PUTENV documentation to flang/docs/Intrinsics.md. Add functional and
semantic unit tests.

This PR adds the intrinsic `unlink` to flang. 

## Test plan
- Added two codegen unit tests and ensured flang-check continues to
pass.
- Manually compiled and ran the example from the documentation.

Implement GNU extension intrinsic HOSTNM, both function and subroutine
forms. Add HOSTNM documentation to `flang/docs/Intrinsics.md`. Add
lowering and semantic unit tests.

(This change is modeled after GETCWD implementation.)

Mostly mechanical changes in preparation of extracting the Flang-RT
"subproject" in #110217. This PR intends to only move pre-existing files
to the new folder structure, with no behavioral change. Common files
(headers, testing, cmake) shared by Flang-RT and Flang remain in
`flang/`.

Some cosmetic changes and files paths were necessary:
* Relative paths to the new path for the source files and
`add_subdirectory`.
 * Add the new location's include directory to `include_directories`
* The unittest/Evaluate directory has unitests for flang-rt and Flang. A
new `CMakeLists.txt` was introduced for the flang-rt tests.
 * Change the `#include` paths relative to the include directive
 * clang-format on the `#include` directives
* Since the paths are part if the copyright header and include guards, a
script was used to canonicalize those
* `test/Runtime` and runtime tests in `test/Driver` are moved, but the
lit.cfg.py mechanism to execute the will only be added in #110217.