x86, libgcc: Implement ia32 basic heap trampoline [PR113855].

The initial heap trampoline implementation was targeting 64b
platforms.  As the PR demonstrates this creates an issue where it
is expected that the same symbols are exported for 32 and 64b.

Rather than conditionalize the exports and code-gen on x86_64,
this patch provides a basic implementation of the IA32 trampoline.

This also avoids potential user confusion, when a 32b target has
64b multilibs, and vice versa; which is the case for Darwin.

	PR target/113855

gcc/ChangeLog:

	* config/i386/darwin.h (DARWIN_HEAP_T_LIB): Moved to be
	available to all sub-targets.
	* config/i386/darwin32-biarch.h (DARWIN_HEAP_T_LIB): Delete.
	* config/i386/darwin64-biarch.h (DARWIN_HEAP_T_LIB): Delete.

libgcc/ChangeLog:

	* config.host: Add trampoline support to x?86-linux.
	* config/i386/heap-trampoline.c (trampoline_insns): Provide
	a variant for IA32.
	(union ix86_trampoline): Likewise.
	(__gcc_nested_func_ptr_created): Implement a basic trampoline
	for IA32.