[zx] Magenta -> Zircon

The Great Renaming is here!

Change-Id: I3229bdeb2a3d0e40fb4db6fec8ca7d971fbffb94

[syscalls] stop using deprecated mx_exit()

Change-Id: I8ef16913797e0ccc8563520d90769f68a999ab3c

[ulib][magenta][runtime][musl] Give system calls _mx_* names, use them in musl

The mx_* name space is not kosher for the standard C library to use
for its own purpose. A conforming standard C program can define its
own functions called mx_* and this must not break the C library.

Give all the system call entry points in the vDSO names starting
with _mx_, with the old mx_ names as aliases. Use only the _mx_
names in runtime and musl.

Change-Id: Id01b57b87c0bb91733e9492159401ff019943822

Make abort() crash instead of just exiting.

Change-Id: Iae6ac563ea1f9973fe1645b381fa5de70f689757

[syscalls][api] rename _magenta_*() to mx_*()

The _magenta_ prefix is pretty unwieldy.
This change simplifies that a bit.

This was done mechanically, and boots, runs, and passes tests
on qemu arm64, amd64, and NUC.

Change-Id: I8eb07db6ed6f4b8cac96344cc4ad65ce3f85f917

[magenta] Initial commit