[gccgo] A type assertion with nil does not succeed

[Ian Lance Taylor]

When doing a type assertion such as
    if v, ok := x.(t); ok { }
where x is nil, the type assertion is not considered to succeed.  This
is true even though v := x.(t) does not cause a panic.  This patch
implements that for gccgo.  Committed to gccgo branch.

Ian

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