[zx] Magenta -> Zircon

The Great Renaming is here!

Change-Id: I3229bdeb2a3d0e40fb4db6fec8ca7d971fbffb94

[scripts] Search for bash with the env(1) trick.

This permits Magenta to build properly on systems where bash is not the default
shell and/or is installed in locations other than /bin.

Change-Id: Ic95e215c5d28b1a6e653bec53283540868c628f8

[scripts] Add arm64 bisect (prebuilt) script

Now the top scripts have the arch in the name

- bisect-prebuilts-x86-64
- bisect-prebuilts-arm64

Change-Id: Idfc68b04352f85ec7fdb154ff9828d98477b2f98