[dev.typeparams] reflect: support big endian architectures in callMethod

Currently, callMethod has some ABI translation code that is not agnostic
of endianness. This change rectifies that by adding a method to
internal/abi.RegArgs for safely returning an offset into a register slot
that's endianness-dependent.

No tests for this because it's just best-effort. There's no actual way
to test this because we don't support a register ABI on any big endian
architectures yet.

Change-Id: Ic68d9ee1bfdea0fc2992d467d749e2b083e92de3
Reviewed-on: https://go-review.googlesource.com/c/go/+/328348


Trust: Michael Knyszek <mknyszek@google.com>
Run-TryBot: Michael Knyszek <mknyszek@google.com>
TryBot-Result: Go Bot <gobot@golang.org>
Reviewed-by: Cherry Mui <cherryyz@google.com>