... instead of BPF: 'error: BPF does not support dynamic stack allocation', and
nvptx: 'sorry, unimplemented: target cannot support alloca'.

	gcc/
	* config/bpf/bpf.md (define_expand "allocate_stack"): Emit
	'sorry, unimplemented: dynamic stack allocation not supported'.
	* config/nvptx/nvptx.md (define_expand "allocate_stack")
	[!TARGET_SOFT_STACK && !(TARGET_PTX_7_3 && TARGET_SM52)]: Likewise.
	gcc/testsuite/
	* gcc.target/bpf/diag-alloca-1.c: Adjust 'dg-message'.
	* gcc.target/bpf/diag-alloca-2.c: Likewise.
	* gcc.target/nvptx/alloca-1-sm_30.c: Likewise.
	* gcc.target/nvptx/vla-1-sm_30.c: Likewise.
	* lib/target-supports.exp (proc check_effective_target_alloca):
	Adjust comment.