Summary:
This pull request replaces the last remaining Unix headers in `JSBigString` with their equivalent Folly Portability headers, and replaces the calls to `getpagesize()` with `sysconf(_SC_PAGESIZE)` since Folly Portability is missing that function.
The work to get this building on windows was mostly done by acoates-ms, this pull request just adds the finishing touches.
## Changelog:
[General] [Fixed] - Fixed `JSBigString` not compiling on Windows due to Unix-specific headers
Pull Request resolved: https://github.com/facebook/react-native/pull/26826
Test Plan: Compiled with Clang and with MSVC (2017)
Differential Revision: D17903214
Pulled By: cpojer
fbshipit-source-id: 230f8fb410fa81d8f13d8b6ccf1147cfc70358bf
80857f295c