Port fbstring_core to big-endian architectures.

Summary: There's 2 ways this could be implemented - either as a series of preprocessor blocks depending on target architecture (as I have implemented it here), or by encapsulating access to MediumLarge::capacity_ within getters/setters as in a similar manner to setSmallSize() and smallSize(). The first option makes the code a bit harder to read, but the second option changes the existing control flow a bit which could slightly alter performance.

I opted for the first so as to keep the existing amd64 flow untouched, but can easily change the pull request to the second option to keep code readability a priority.
Closes https://github.com/facebook/folly/pull/244

Reviewed By: @Gownta

Differential Revision: D2306568

Pulled By: @JoelMarcey

folly/docs/FBString.md

@@ -9,8 +9,8 @@ allocator. In particular, `fbstring` is designed to detect use of
 jemalloc and cooperate with it to achieve significant improvements in
 speed and memory usage.
 
-`fbstring` supports x32 and x64 architectures. Porting it to big endian
-architectures would require some changes.
+`fbstring` supports 32- and 64-bit and little- and big-endian
+architectures.
 
 ### Storage strategies
 ***