Here's a patch that folds PGO into PCbuild\build.bat (removing PCbuild\build_pgo.bat) and defaults to using "python -m test --pgo" for training.  This should apply to all branches, but will need some slight adjustment for 2.7 (namely, s/-m test/-m test.regrtest/).  It might also be nice to leave a stub build_pgo.bat on 2.7 and 3.5 in case anyone actually uses it.

This patch also switches the default build environment to 'amd64' rather than 'x86_amd64' for -p x64 when it looks like it's running on a 64 bit OS.  PGO requires 'amd64' for 64 bit builds, and if x86_amd64 is available, 'amd64' should be as well.  'x86_amd64' is still used for non-PGO builds when -p x64 is specified and it looks like a 32 bit OS.  I considered switching the default platform target to x64 when running on 64 bit, but that's a separate issue.  It uses "%ProgramFiles(x86)"=="" to detect 32-bit, and gives a helpful message in the case of a false positive on that check.