Generic implementation of common syscall, interrupt and exception

entry/exit functionality based on the recent X86 effort to ensure
correctness of entry/exit vs. RCU and instrumentation.

As this functionality and the required entry/exit sequences are not
architecture specific, sharing them allows other architectures to benefit
instead of copying the same code over and over again.

This branch was kept standalone to allow others to work on it. The
conversion of x86 comes in a seperate pull request which obviously is based
on this branch.
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