folly: remove template argument to std::abs to fix build with libc++

folly: remove template argument to std::abs to fix build with libc++

Summary:
libc++ doesn't implement `std::abs` as template.

```lang=bash
./../folly/fibers/Fiber.h:68:34: error: unexpected type name 'intptr_t': expected
      expression
    const size_t size = std::abs<intptr_t>(
                                 ^
```

Reviewed By: markisaa

Differential Revision: D3645584

fbshipit-source-id: a2d672137f110b975412a94ae4c6d00da0bd0d43