Update platform functions to use major and minor version number on FreeBSD

[wfurt]

this is follow-up on dotnet/core-setup#3228.

right now we get this on FreeBSD running dotnet --info

Runtime Environment:
OS Name: FreeBSD
OS Version: 11
OS Platform: FreeBSD
RID: freebsd.11-x64

To be consistent with other platforms this should be updated to retune something like 11.2 for version (or 12.0) and rid should be freebsd.11.2-64

cc: @bartonjs

[dagood]

Based on dotnet/core-setup#3228, this is a host change. (Setting label.)

[arrowd]

Haskell Stack doesn't discriminate between minor version. Neither should .NET, because minor releases are guaranteed to be ABI compatible.