[roll] Roll fuchsia [utest] Use ELF loading in restricted mode tests

Modifies the restricted mode tests to load a separate ELF binary instead
of jumping into code linked into the test binary itself. The actual
contents of the code executed in restricted mode have not changed;
they've simply been moved to their own binary. This will allow us to add
tests for ARM32 machines on ARM64 hosts more easily.

To make this work, I needed to add some hidden labels in the
restricted-blob assembly code to avoid GOT indirections (which caused
link failures in RISC-V). While the failure was only observed on RISC-V,
it is theoretically possible on other architectures, so I switched to
using the same structure everywhere.

This change also renames the "vectab" function to "restricted_exit" to
make the purpose of the function more clear and to match other
restricted mode tests.

Original-Bug: 372499348
Original-Reviewed-on: https://fuchsia-review.googlesource.com/c/fuchsia/+/1157128
Original-Revision: bbe0664791c55b17ae2ff097c5dd6d27b7ca9210
GitOrigin-RevId: 05cb6a91c8a1a6b886c2344f46ecaec26fb1d122
Change-Id: I99208f8000b1efd51d9020144f915b35d288a813