[zx] Magenta -> Zircon

The Great Renaming is here!

Change-Id: I3229bdeb2a3d0e40fb4db6fec8ca7d971fbffb94

[build] Avoid bash-4 features in scripts/gen-vdso-valid-sysret.sh

Associative arrays are a bash v4 feature not present in the bash
shell shipped on OSX. Use ugly 1970s sh techniques instead!

Change-Id: I4f9ddbf9f610af23f1a8ddff721c082130305ad0

[kernel][syscalls] Enforce vDSO use for system calls

This uses the tracking of the vDSO code mapping address in each
process to enforce that every system call instruction entering the
kernel must be from the proper vDSO stub code for the system call
being made.

Change-Id: Idcf124f7bfa8c83c46acec89c611ac47ec8ccad1