[gdb/testsuite] Add gdb.testsuite/version-compare.exp

Add a test-case gdb.testsuite/version-compare.exp that excercises proc
version_compare, and a note to proc version_compare that it considers
v1 < v1.0 instead of v1 == v1.0.

Tested on x86_64-linux.

Approved-By: Tom Tromey <tom@tromey.com>