gdbsupport: remove FUNCTION_NAME

gdbsupport: remove FUNCTION_NAME

__func__ is standard C++11:

    https://en.cppreference.com/w/cpp/language/function

Also, in C++11, __func__ expands to the demangled function name, so the
mention in the comment above FUNCTION_NAME doesn't apply anymore.
Finally, in places where FUNCTION_NAME is used, I think it's enough to
print the function name, no need to print the whole signature.
Therefore, I propose to just remove FUNCTION_NAME and update users to
use the standard __func__.

Change-Id: I778f28155422b044402442dc18d42d0cded1017d