[roll] Roll fuchsia [userboot] Link userboot as a standalone static PIE

This changes userboot to be a "normal" static PIE, doing its own
self-relocation with full dynamic linking against the vDSO.  It's
still entirely statically-linked except for the vDSO and still
doesn't have much libc functionality.  But it's now built in the
user.basic environment instead of a custom userboot environment
and much simplified by leveraging elfldltl static PIE support.

This ends the "special relationship" between userboot and vDSO
implementations.  The userboot binary is no longer tightly
coupled to the vDSO's implementation.  It's still coupled to the
kernel implementation but only by its precise expectations about
the layout of its bootstrap message from the kernel.

The userboot code is now relieved of all its restrictions on
having writable data and dynamic relocations.  As it gets much
easier to work in the userboot code, it will be easier to loosen
the coupling with the kernel further.  The remaining difficulties
stem from missing libc support for "fully normal" static PIEs,
which will come later.

Original-Reviewed-on: https://fuchsia-review.googlesource.com/c/fuchsia/+/1268024
Original-Revision: e205d2b3a6459762ffda4b4a7102b27077c3deb4
GitOrigin-RevId: 64b34582cb9a76914084c05bbf654705973693b0
Change-Id: Ib67849026e632130bbd209415cae30f688197f84