How to pass the name of environment variable as an argument?

You need delayed expansion to output a variable referenced by a cmd line argument.
This batch uses two different methods for this,

• a pseudo call with doubled percent signs (which forces the parser to evaluate the line twice, on first pass reducing two %% to one in 2nd pass the other vars are already expanded) and
• using the ! instead the normal single percent sign with a prior command setlocal enabledelayedexpansion

:: Q:\Test\2018\05\10\SO_50280684.cmd
@Echo off&SetLocal EnableDelayedExpansion
Echo(%1
Echo(!%1!
Call Echo(%%%1%%

I use the command separator ( instead off a space to suppress an Echo is off message incase of an empty var.

Sample with command

>SO_50280684.cmd PROCESSOR_ARCHITECTURE
PROCESSOR_ARCHITECTURE
AMD64
AMD64