libgcc: support heap-based trampolines

Add support for heap-based trampolines on x86_64-linux, aarch64-linux,
and x86_64-darwin. Implement the __builtin_nested_func_ptr_created and
__builtin_nested_func_ptr_deleted functions for these targets.

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

libgcc/ChangeLog:

	* libgcc2.h (__builtin_nested_func_ptr_created): Declare.
	(__builtin_nested_func_ptr_deleted): Declare.
	* libgcc-std.ver.in: Add the new symbols.
	* config/aarch64/heap-trampoline.c: Implement heap-based
	trampolines for aarch64.
	* config/aarch64/t-heap-trampoline: Add rule to build
	config/aarch64/heap-trampoline.c
	* config/i386/heap-trampoline.c: Implement heap-based
	trampolines for x86_64.
	* config/i386/t-heap-trampoline: Add rule to build
	config/i386/heap-trampoline.cc
	* config.host: Handle --enable-heap-trampolines on
	x86_64-*-linux*, aarch64-*-linux*, x86_64-*-darwin*.