build,win: limit maxcpucount to 2 for MSBuild

MSBuild invokes cl.exe with /MP (set in common.gypi), making it
compile sources in parallel using a number of internal processes
equal to the number of effective processors. MSBuild /m uses a
similar mechanism, so the number of compiler processes can grow to
the number of effective processors squared.

This limits MSBuild to 2 processes, to still use some parallelization
while requiring less memory. Cl.exe is still invoked with /MP, thus
the maximum number of processes is limited to twice the number of
effective processors.

PR-URL: https://github.com/nodejs/node/pull/12184


Reviewed-By: Refael Ackermann <refack@gmail.com>
Reviewed-By: Ben Noordhuis <info@bnoordhuis.nl>