-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.

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