liblog: test: pmsg overhead measurement

This test tells us that scatter-gather (writev instead of write)
carries a small ~2% penalty. Unaligned buffer carry a similar
additional penalty. On N9 it takes 3us to 22us _just_ to write
the pmsg logs, depending on size. Some assumptions about the
socket read and write performance for the main logging can be made
from tracking these results and should improve design decisions.

Bug: 18771697
Bug: 23685592
Change-Id: Id4d64c449140e4f39078c62b0097e403df91fe0c