[Darwin] Suppress emitting empty ctor/dtor sections.

[Darwin] Suppress emitting empty ctor/dtor sections.

Older versions of GCC emit empty .constructor/.destructor sections
whenever building for C++. In fact, these sections are only used for
kernel mode code - so don't emit them unless that's what we're
building.

gcc/ChangeLog:

2019-10-12  Iain Sandoe  <iain@sandoe.co.uk>

	* config/darwin.c (darwin_file_end): Only emit empty CTOR/DTOR
	sections when building kernel extension code.

From-SVN: r276923