The problem is link time code generation. With link time optimization[0], which is a prerequisite for profile guided optimization, cl just compiles the source into some intermediate representation. Actual code generation is all done in link.exe at the final link. And /machine:x86 doesn't work with link.exe when using -GL. The x64 link.exe is not capable of generating x86 binaries with LTO.

[0] http://msdn.microsoft.com/en-us/library/0zza0de8%28v=vs.100%...

(Mozilla is building with 2010 now.)