Speed-up JIT dump

Speed-up JIT dump

Add a config option that allows routing JIT's stdout output to a
specified file. jitStartup disables stdio buffering on its stdout
created stream and that makes large dumps (such as the ones generated
by jit-diff) 2-4x slower. Simply enabling buffering is somewhat
problematic, it would require occasional flushing to avoid losing
output in the case of a crash and can also result in mingled output
if crossgen decides to print something via its own stdout stream.

file

fix

Commit migrated from https://github.com/dotnet/coreclr/commit/193e3134e3726b933111b46617be9bee05084ee2