fix: don't clobber saved frame pointer in arm64 assembly functions

[nsrip-dd]

The arm64 neon assembly functions in this repository overwrite the frame
pointer saved by their callers, leading to crashes from the Go runtime
execution tracer and profilers which use frame pointer unwinding. For
historical reasons, on arm64 Go functions save the caller's frame
pointer register (x29) one word below their stack frame. See
go.dev/s/regabi#arm64-architecture. The assembly functions here,
translated from C compiler output, save values at the top of their
frame, and overwrite the frame pointer saved by the caller. We can fix
this by decrementing the stack pointer past where that frame pointer is
saved before saving anything on the stack.

Fixed with this sed script on my macos laptop + manual cleanup to match
indentation:

/stp[\t ]*x29/i\
	// The Go ABI saves the frame pointer register one word below the \
	// caller's frame. Make room so we don't overwrite it. Needs to stay \
	// 16-byte aligned \
	SUB $16, RSP

/ldp[\t ]*x29/a\
	// Put the stack pointer back where it was \
	ADD $16, RSP

Ran the script from the root of this repository with

find . -name '*_arm64.s' -exec sed -f fix.sed -i '' {} +

Then manually inspected the assembly for missing SUBs/ADDs at the
beginning of functions and prior to returns.

Fixes #150