Find first non-zero jit debug entry.

Sometimes a process will have multiple shared libraries loaded
that have defined __jit_debug_descriptor. Specifically, art testing
will load libart.so and libartd.so, which would have broken unwinding
through jit'd code for art testing if libart.so winds up being found first.
In order to avoid duplicating the code for the linker to figure out
which one is live, change the algorithm to find the first non-zero
first_entry_ set in __jit_debug_descriptor.

Bug: 68396769

Test: Passes unit tests.
Test: Verified this can unwind 137-cfi on arm/arm64.
Change-Id: Ic4d403065d2c6f22476ef0171e7add17cd1464cd