X86Subtarget.h could be beautified


when reading X86Subtarget.h the methods seem a bit disordered, therefore I would
propose to sort them new:
-cpu features (e.g. hasSSE1())
-os types (e.g. isTargetDarwin())
-object types (e.g. isTargetELF())
-callconv related functions (e.g. isTargetWin64(), consider renaming to isCallConvWin64(),
-pic functions

perhaps my problem of generating elf objects on windows can be easily solved by creating
a new os type, just like mingw and cycwin, and patching X86Subtarget.h accordingly
(of course only in my private copy, i doubt it will be of general use)