-fp:fast study continued

MSVC x64 toolchain’s -fp:fast, which can cause LTCG:pgi build running behavior more different with normal, is more sensitive than x86. So, we should test the x64 build as a prototype. (No PGO build and LTCG:pgi build should all pass the UNIT tests?)
According to https://pcxfirefox.wordpress.com/2016/10/16/a-questionable-but-faster-msvc-compiler-switch, we can use some pragma code section to avoid -fp:fast bugs, but this is very difficult checking in Mozilla Unified build. So, we should use FILES_PER_UNIFIED_FILE = 1 and change UNIFIED_SOURCES to SOURCES in order to find the code section.

There will be no 50.0 released

Because there will certainly be a 50.0.1.

In this period, I change my build machine to i7 6700/1060/SSD, so I must modify my all auto build scripts and update scripts. Maybe I update 50.0 edition, but this is only for testing whether my scripts are OK. I won’t guarantee the edition is stable.