target: Support heap-based trampolines

Enable -ftrampoline-impl=heap by default if we are on macOS 11
or later.

Co-Authored-By: Maxim Blinov <maxim.blinov@embecosm.com>
Co-Authored-By: Francois-Xavier Coudert <fxcoudert@gcc.gnu.org>
Co-Authored-By: Iain Sandoe <iain@sandoe.co.uk>

gcc/ChangeLog:

	* config.gcc: Default to heap trampolines on macOS 11 and above.
	* config/i386/darwin.h: Define X86_CUSTOM_FUNCTION_TEST.
	* config/i386/i386.h: Define X86_CUSTOM_FUNCTION_TEST.
	* config/i386/i386.cc: Use X86_CUSTOM_FUNCTION_TEST.