Fix Windows context to Unix context translation on AMD64 (dotnet/coreclr#6027)

Fix Windows context to Unix context translation on AMD64 (dotnet/coreclr#6027)

There was a bug in the context translation between the Windows context
and Unix context on AMD64 caused by the fact that the Unix context
gregs array contains CS, GS and FS in a single field (REG_CSGSFS) and
the MCREG_SegCs accessor macro was incorrectly written to use the
whole field as CS. So writing the CS into the Unix context also
cleared the GS, FS and the topmost 16 bits described as padding.
This issue was exposed on the Linux kernel >= 4.6.0 where the padding
for some reason was not zero, probably used by the kernel for some
internal purposes.
I have fixed it by changing the accessor to modify only the 16 bits
corresponding to the CS.
I have also changed the code in the inject_activation_handler to
save cycles and not to copy the Windows context back to the Unix one
in case the activation function was not called and so the context
was not possibly changed.

Commit migrated from https://github.com/dotnet/coreclr/commit/56ab756b0e9afef24cf9151a4a43f23e8d78de29