PR go/90635
    libgo: correct typo in USE_LIBFFI AM_CONDITIONAL
    
    Only affects the case of passing --without-libffi to configure.
    
    Fixes https://gcc.gnu.org/PR90635
    
    Reviewed-on: https://go-review.googlesource.com/c/gofrontend/+/178998

From-SVN: r271640