Add support for SVE stack clash probing.

Add support for SVE stack clash probing.

This patch adds basic support for SVE stack clash protection.
It is a first implementation and will use a loop to do the
probing and stack adjustments.

An example sequence is:

        .cfi_startproc
        mov     x15, sp
        cntb    x16, all, mul #11
        add     x16, x16, 304
        .cfi_def_cfa_register 15
.SVLPSPL0:
        cmp     x16, 61440
        b.lt    .SVLPEND0
        sub     sp, sp, 61440
        str     xzr, [sp, 0]
        sub     x16, x16, 61440
        b      .SVLPSPL0
.SVLPEND0:
        sub     sp, sp, x16
        .cfi_escape 0xf,0xc,0x8f,0,0x92,0x2e,0,0x8,0x58,0x1e,0x23,0xb0,0x2,0x22

for a 64KB guard size, and for a 4KB guard size

        .cfi_startproc
        mov     x15, sp
        cntb    x16, all, mul #11
        add     x16, x16, 304
        .cfi_def_cfa_register 15
.SVLPSPL0:
        cmp     x16, 3072
        b.lt    .SVLPEND0
        sub     sp, sp, 3072
        str     xzr, [sp, 0]
        sub     x16, x16, 3072
        b       .SVLPSPL0
.SVLPEND0:
        sub     sp, sp, x16
        .cfi_escape 0xf,0xc,0x8f,0,0x92,0x2e,0,0x8,0x58,0x1e,0x23,0xb0,0x2,0x22

This has about the same semantics as alloca, except we prioritize the common case
where no probe is required.  We also change the amount we adjust the stack and
the probing interval to be the nearest value to `guard size - abi buffer` that
fits in the 12-bit shifted immediate used by cmp.

While this would mean we probe a bit more often than we require, in practice the
amount of SVE vectors you'd need to spill is significant. Even more so to enter the
loop more than once.

gcc/

	PR target/86486
	* config/aarch64/aarch64-protos.h (aarch64_output_probe_sve_stack_clash): New.
	* config/aarch64/aarch64.c (aarch64_output_probe_sve_stack_clash,
	aarch64_clamp_to_uimm12_shift): New.
	(aarch64_allocate_and_probe_stack_space): Add SVE specific section.
	* config/aarch64/aarch64.md (probe_sve_stack_clash): New.

gcc/testsuite/

	PR target/86486
	* gcc.target/aarch64/stack-check-prologue-16.c: New test
	* gcc.target/aarch64/stack-check-cfa-3.c: New test.
	* gcc.target/aarch64/sve/struct_vect_24.c: New test.
	* gcc.target/aarch64/sve/struct_vect_24_run.c: New test.

From-SVN: r264749