Merge pull request #2612 from xmcclure/largestring

Do not run 4GB string tests on low memory systems (fixes #38392)

"Low memory" here is defined as 32-bit, less than 6GB of physical RAM, or mobile (since mobile lacks the class for querying physical RAM).

Without this patch, one of the corlib tests allocates 4 GB of RAM, which causes OOM crashes on low-memory Jenkins instances and also mobile phones.