[eval] Fix haxe.Int64 to native Int64 conversion

[Aurel300]

The problem occurs when low is >= 0x80000000, because Int32.to_int64 sign-extends the low number. The high 32 bits are then all set, so after GInt64.logor the high value is completely lost.

high (int32):   cafecafe
low (int32):    80000001
high (int64):   cafecafe00000000
low (int64):    ffffffff80000001
result:         ffffffff80000001

This fix bitmasks the sign-extended low value to only keep the low 32 bits again.