When MAKEOBJDIRPREFIX points to a case-insensitive file system, the
build can break when different source files create the same target
files (case-insensitivity speaking). This is the case for object
files compiled with -fpic and shared libraries. The former uses
an extension of ".So", and the latter an extension ".so". Rename
shared object files from *.So to *.pico to match what NetBSD does.

Missed in r306297

MFC after: 1 month
Sponsored by: Bracket Computing
Differential Revision: https://reviews.freebsd.org/D7906