Generate built-in dispatch.ll from C source

[nurmukhametov]

It looks like there is no real reason to have dispatch*.ll files manually generated from dispatch.c source. It can be generated during ISPC build process.

Due to missing tests for dispatch logic, it depends on #2815

[dbabokin]

Not a call to action, but just to add broader context here. The logic of dispatch module exists in ISPC compiler in 3 different incarnations:

• dispatch.ll - library code that is injected into user binary when auto-dispatch feature is used (i.e. multiple targets are specified in the command line)
• the code checking the ISA of the host where ISPC binary is executed (to determine the target when --target=host is specified) - it's part of the compiler itself. lGetSystemISA() function in ispc.cpp.
• the code of check_isa.cpp - which is standalone utility reporting the ISA of the machine where it is executed on.

I thought about unifying all there 3 places, but they have different requirements and probably unification introduces more problems than brings value.