gcc: testsuite: Fix builtin-speculation-overloads[14].C testism

gcc: testsuite: Fix builtin-speculation-overloads[14].C testism

When making warnings trigger a failure in template substitution I
could not find any way to trigger the warning about builtin speculation
not being available on the given target.

Turns out I misread the code -- this warning happens when the
speculation_barrier pattern is not defined.

Here we add an effective target to represent
"__builtin_speculation_safe_value is available on this target" and use
that to adjust our test on SFINAE behaviour accordingly.
N.b. this means that we get extra testing -- not just that things work
on targets which support __builtin_speculation_safe_value, but also that
the behaviour works on targets which don't support it.

Tested with AArch64 native, AArch64 cross compiler, and RISC-V cross
compiler (just running the tests that I've changed).

Ok for trunk?

gcc/testsuite/ChangeLog:

	PR target/117991
	* g++.dg/template/builtin-speculation-overloads.def: SUCCESS
	argument in SPECULATION_ASSERTS now uses a macro `true_def`
	instead of the literal `true` for arguments which should work
	with `__builtin_speculation_safe_value`.
	* g++.dg/template/builtin-speculation-overloads1.C: Define
	`true_def` macro on command line to compiler according to the
	effective target representing that
	`__builtin_speculation_safe_value` does something on this
	target.
	* g++.dg/template/builtin-speculation-overloads4.C: Likewise.
	* lib/target-supports.exp
	(check_effective_target_speculation_barrier_defined): New.

Signed-off-by: Matthew Malcomson <mmalcomson@nvidia.com>