[nfc][libomptarget] Reorganise support header
All functions defined in support implementation are now declared in support.h
Reordered functions in support implementation to match the sequence in support.h
Added include guards to support.h
Added #include interface to support.h to provide kmp_Ident declaration
Move supporti.h to support.cu and s/INLINE/EXTERN/g
Add remaining includes to support.cu
A minor side effect is to change the name mangling of the support functions to
extern "C". If this matters another macro along the lines of INLINE/EXTERN
can be added - perhaps DEVICE as that's the obvious implementation.
The device RTL is not consistent here (__XXX and _XXX are used) and it is different from LLVM. I'd vote for LLVM style, thus OMPTARGET_SUPPORT_H