Add USDT header file to folly

Add USDT header file to folly

Summary:
Derived from `sdt.h` in [[ https://sourceware.org/systemtap/ | SystemTap ]]
Performed changes and clean-ups including:
- Get rid of the `sdt-config.h`.
- Get rid of "Running in Assembler" code.
- Replace the `DTRACE_PROBE_1, ... DTRACE_PROBE_N` Macro family with a single Macro using the "count Macro arguments" trick
- Get rid of argument signed-ness calculation. A **lot** of painful code were there only for this, and it has no real usage except for displaying purposes.
- Re-structure the helper methods (Macros) so that the logic of the code is a bit more readable.
- Add some comments so that it would be easier for users and future developers to understand what this header file is actually doing.
- Update license banner.

Reviewed By: meyering

Differential Revision: D3792966

fbshipit-source-id: 564a1011f7946d30be78191f334e072d94fa264e