This libgo patch by Funan Zeng synchronizes the handling of empty
struct fields between the Go frontend and the libgo FFI code.  In the
compiler the logic for allocating one byte for the last field of a
struct is:
1. the last field has zero size
2. the struct itself does not have zero size
3. the last field is not blank
This patch adds the last two conditions to runtime.structToFFI.  This
is for https://go.dev/issue/55146.  Bootstrapped and ran Go testsuite
on x86_64-pc-linux-gnu.  Committed to mainline.

Ian