Christian Blichmann 6a1e4b881c Introduce config header to centralize CPU architecture checks
This allows us to remove some uses of macros.

Related changes:
- Make it clear that we support hosting sandboxed binaries from 64-bit
  processes only. CPU architectures are x86-64 and POWER64 (little endian).
- Introduced CPU architecture macros, abstracting away compiler specifics

PiperOrigin-RevId: 330918134
Change-Id: Ife7ad5f14723eec9f68055127b0583b8aecd38dd
2020-09-10 05:48:00 -07:00
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