Simpler fix for Dangling Pointer issue

[mikee47]

Whilst using a function to 'hide' the stack return appears to work, integration tests fail in a couple of scenarios (FlashString, RapidXML) because of the way stack space gets used. For Host builds, the most pragmatic solution is to make _F(s) return s directly. That means it gives a const char* rather than char*, and any attempt to pass this to a function requiring a regular char* will fail.

[what-the-diff]

PR Summary

• Removal of smg_return_local function
  The function smg_return_local has been removed from the FakePgmSpace.cpp file. This means we've trimmed off unnecessary code.

• Modification of smg_return_local macro
  The smg_return_local macro found in FakePgmSpace.h file has been adjusted. Instead of invoking a function, it directly delivers the buffer, which should streamline the process, thus making it a bit more efficient.

• Addition of _F(str) (str) definition
  A new definition has been added to the FakePgmSpace.h file: _F(str) (str). This can be invoked to simplify specific string-related operations in the code, adding an easy-to-use way to manipulate strings.