ARM64: Fix Android linker issues

[RalfKornmannEnvision]

• Add implementation for unused function in libunwind that is not removed by the Android linker
• Fix the data type for the pointer to return address macro as the android linker did not relocate the full address with the wrong type
• Make all assembler functions hidden. This way they are not added to the GOT. This forces the linker to resolve all calls to the functions itself instead of using the PLT. Using the PLT will trash X16&X17. Some of the functions implemented in assembler require that these two register are not changed. Beside of this using the PLT adds more overhead.