[cmake] Update SOVERSION for the new versioning scheme
[cmake] Update SOVERSION for the new versioning scheme

Update SOVERSION to use just the major version number rather than
major+minor, to match the new versioning scheme where only major is used
to indicate API/ABI version.

Since two-digit SOVERSIONs were introduced post 3.9 branching, this
change does not risk any SOVERSION collisions. In the past,
two-component X.Y SOVERSIONs were shortly used but those will not
interfere with the new ones since the new versions start at 4.

Differential Revision: https://reviews.llvm.org/D28730

git-svn-id: 0ff597fd157e6f4fc38580e8d64ab130330d2411@292255 91177308-0d34-0410-b5e6-96231b3b80d8

GPUOpen-Drivers/llvm,apple/swift-llvm,llvm-mirror/llvm,GPUOpen-Drivers/llvm,GPUOpen-Drivers/llvm,apple/swift-llvm,llvm-mirror/llvm,llvm-mirror/llvm,llvm-mirror/llvm,apple/swift-llvm,apple/swift-llvm,llvm-mirror/llvm,llvm-mirror/llvm,GPUOpen-Drivers/llvm,apple/swift-llvm,apple/swift-llvm,llvm-mirror/llvm,llvm-mirror/llvm,apple/swift-llvm,apple/swift-llvm,GPUOpen-Drivers/llvm,GPUOpen-Drivers/llvm,GPUOpen-Drivers/llvm,GPUOpen-Drivers/llvm,llvm-mirror/llvm