Use assembly stub to take the address of assembly functions

Go 1.17 is adding a new register-based calling convention [1] ("ABIInternal"),
which used is when calling between Go functions. Assembly functions are still
written using the old ABI ("ABI0"). That is, they still accept arguments on the
stack, and pass arguments to other functions on the stack. The call rules look
approximately like this:

1. Direct call from Go function to Go function: compiler emits direct
   ABIInternal call.
2. Indirect call from Go function to Go function: compiler emits indirect
   ABIInternal call.
3. Direct call from Go function to assembly function: compiler emits direct
   ABI0 call.
4. Indirect call from Go function to assembly function: compiler emits indirect
   ABIInternal call to ABI conversion wrapper function.
5. Direct or indirect call from assembly function to assembly function:
   assembly/linker emits call to original ABI0 function.
6. Direct or indirect call from assembly function to Go function:
   assembly/linker emits ABI0 call to ABI conversion wrapper function.

Case 4 is the interesting one here. Since the compiler can't know the ABI of an
indirect call, all indirect calls are made with ABIInternal. In order to
support indirect ABI0 assembly function calls, a wrapper is generated that
translates ABIInternal arguments to ABI0 arguments, calls the target function,
and then converts results back.

When the address of an ABI0 function is taken from Go code, it evaluates to the
address of this wrapper function rather than the target function so that later
indirect calls will work as expected.

This is normally fine, but gVisor does more than just call some of the assembly
functions we take the address of: either noting the start and end address for
future reference from a signal handler (safecopy), or copying the function text
to a new mapping (platforms).

Both of these fail with wrappers enabled (currently, this is Go tip with
 GOEXPERIMENT=regabiwrappers) because these operations end up operating on the
wrapper instead of the target function.

We work around this issue by taking advantage of case 5: references to assembly
symbols from other assembly functions resolve directly to the desired target
symbol. Thus, rather than using reflect to get the address of a Go reference to
the functions, we create assembly stubs that return the address of the
function. This approach works just as well on current versions of Go, so the
change can be made immediately and doesn't require any build tags.

[1] https://go.googlesource.com/go/+/refs/heads/master/src/cmd/compile/abi-internal.md

PiperOrigin-RevId: 368505655

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