[Flang][OpenMP][Runtime] Minor Flang runtime for OpenMP AMDGPU modifications (#152631)

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