main/unixbench: add aport for a benchmarking tool (!145)

Adds an aport for unixbench and a wrapper so it can be started with
ubench without requiring write permissions in /usr. To launch the
benchmark on a phone and have it finish in a reasonable amount of time:

$ ubench dhry whets
Output on an i7-7700HQ in the pmbootstrap chroot:
Dhrystone 2 using register variables      146273178.2 lps   (10.0 s, 7 samples)
Double-Precision Whetstone                    44390.3 MWIPS (9.7 s, 7 samples)

System Benchmarks Partial Index              BASELINE       RESULT    INDEX
Dhrystone 2 using register variables         116700.0  146273178.2  12534.1
Double-Precision Whetstone                       55.0      44390.3   8071.0
                                                                   ========
System Benchmarks Index Score (Partial Only)                        10057.9

This is a neat way to get relative performance between devices and
testing performance for upstream/downstream kernels.