bpf: liberate R9 for general register allocation

bpf: liberate R9 for general register allocation

We were reserving one of the hard registers in BPF in order to
implement dynamic stack allocation: alloca and VLAs. However, there is
kernel code that has inline assembly that requires all the non-fixed
registers to be available for register allocation.

This patch:

1. Liberates r9 that is now available for register allocation.

2. Adds a check to GCC so it errors out if the user tries to do
   dynamic stack allocation.  A couple of tests are added for this.

3. Changes xbpf so it no longer saves and restores callee-saved
   registers.  A couple of tests for this have been removed.

4. Adds bpf-*-* to the list of targets that do not support alloca in
   target-support.exp.

Tested in host x86_64-linux-gnu and target bpf-unknown-none.

gcc/ChangeLog

	* config/bpf/bpf.md (allocate_stack): Define.
	* config/bpf/bpf.h (FIRST_PSEUDO_REGISTER): Make room for fake
	stack pointer register.
	(FIXED_REGISTERS): Adjust accordingly.
	(CALL_USED_REGISTERS): Likewise.
	(REG_CLASS_CONTENTS): Likewise.
	(REGISTER_NAMES): Likewise.
	* config/bpf/bpf.cc (bpf_compute_frame_layout): Do not reserve
	space for callee-saved registers.
	(bpf_expand_prologue): Do not save callee-saved registers in xbpf.
	(bpf_expand_epilogue): Do not restore callee-saved registers in
	xbpf.

gcc/testsuite/ChangeLog

	* lib/target-supports.exp (check_effective_target_alloca): BPF
	target does not support alloca.
	* gcc.target/bpf/diag-alloca-1.c: New test.
	* gcc.target/bpf/diag-alloca-2.c: Likewise.
	* gcc.target/bpf/xbpf-callee-saved-regs-1.c: Remove test.
	* gcc.target/bpf/xbpf-callee-saved-regs-2.c: Likewise.
	* gcc.target/bpf/regs-availability-1.c: Likewise.