Don't throw exceptions from operator new by default

[earlephilhower]

Default mode (no exceptions) will no longer use the stdc++ library new
allocator which throw()s and error when there is not enough memory. Instead, it will return
nullptr. This is the pre-exceptions-available behavior (2.5.0 and
earlier).

When exceptions are enabled, use the real new and throw exceptions that
can be caught at higher levels, or which will crash the app with an
uncaught exception if they're not handled.

Update to #6309

[d-a-v]

This PR is wrong, as was #6309
When exceptions are disabled:

• operator new: no exception without need for std::nothrow #6309:
  • pros:
    • new can return null
    • in that case constructor is not executed
    • legacy arduino can return null too (but executes constructor in that case)
  • cons:
    • dirty hack
    • require lots of changes, probably in external libraries too
• this PR:
  • pros:
    • new can return null
    • legacy arduino can return null too
  • cons:
    • constructor is always executed (even if this==nullptr) (like legacy arduino does)
• nothing:
  • pros:
    • abort() is called (that's c++ standard with -fno-exceptions)
    • constructor is not executed when nullptr
    • we can remove all check for nullptr when calling new
  • cons:
    • not compatible with arduino legacy
      but majority doesn't care of new return value being nullptr.

[d-a-v]

A failing malloc will return nullptr but constructor will be called. On simple objects this is fine since ram is not modified (no issue). On complex object a segfault(LoadStoreException -> abort) will occur causing an exception.
Never mind, this is both legacy and arduino way (be nice with available ram).