compiler: revert `for package-scope "a = b; b = x" just set "a = x"`

This patch to the Go frontend reverts https://go.dev/cl/245098, which
introduced incorrect initialization ordering.  This patch adjusts the
runtime package to work even with that change reverted.

Original description of change 245098:

    This avoids requiring an init function to initialize the variable.
    This can only be done if x is a static initializer.

    The go1.15rc1 runtime package relies on this optimization.
    The package has a variable "var maxSearchAddr = maxOffAddr".
    The maxSearchAddr variable is used by code that runs before package
    initialization is complete.

This is for https://golang.org/issue/51913.

Bootstrapped and ran Go testsuite on x86_64-pc-linux-gnu.  Committed
to mainline.

Ian