libgo: support gollvm build on arm64 linux

    
    This CL serves as part of an initial change for enabling gollvm
    building on arm64 linux, the rest of the change will be covered by
    another one to the gollvm repo.
    
    Incorporate type definition of 'uint128' to 'runtime' and 'syscall'
    packges, the change is not specific to arm64 linux but made available
    for all platforms.
    
    Verified by building and unit-testing gollvm on linux x86-64 and arm64.
    
    Verified by building and checking gccgo on linux x86-64 and arm64.
    
    Fixes golang/go#33711
    
    Change-Id: I4720c7d810cfd4ef720962fb4104c5641b2459c0

From-SVN: r275919