Fix the microbenchmarks build

[adamsitnik]

reported offline by @kunalspathak

[2021/02/12 01:04:06][INFO] /home/helixbot/work/9CF2092C/w/A7AC08F0/e/src/benchmarks/micro/libraries/System.IO.FileSystem/Perf.FileStream.cs(80,17): error CA1416: This call site is reachable on all platforms. 'FileStream.Lock(long, long)' is unsupported on: 'macos'. [/home/helixbot/work/9CF2092C/w/A7AC08F0/e/src/benchmarks/micro/MicroBenchmarks.csproj]
[2021/02/12 01:04:06][INFO] /home/helixbot/work/9CF2092C/w/A7AC08F0/e/src/benchmarks/micro/libraries/System.IO.FileSystem/Perf.FileStream.cs(82,17): error CA1416: This call site is reachable on all platforms. 'FileStream.Unlock(long, long)' is unsupported on: 'macos'. [/home/helixbot/work/9CF2092C/w/A7AC08F0/e/src/benchmarks/micro/MicroBenchmarks.csproj]
[2021/02/12 01:04:06][INFO]     0 Warning(s)
[2021/02/12 01:04:06][INFO]     2 Error(s)

while testing my change I've realized that the 2.1 and 3.1 builds were broken too, so I've fixed them as well