Remove reliance on SBRK instruction

[ggwpez]

Gray Paper 0.8 removed the SBRK instruction for the hostcall grow_heap. The allocator relies on sbrk here for its initial slab.

picoalloc/src/env/polkavm.rs

Lines 5 to 10 in 5645661

	fn sbrk(size: usize) -> *mut u8 {
	// SAFETY: Allocating memory is always safe.
	unsafe {
	let address;
	core::arch::asm!(
	".insn r 0xb, 1, 0, {dst}, {size}, zero",

I will hack it to make my Service SDK work again with GP0.8, but maybe a better long term fix would be possible.

PS: I just saw the sbrk from musl, so will assume that it always links against that and use it.