RFR (XS) 8211274: x86_32 build failures after JDK-8211029 (Have a common set of enabled warnings for all native libraries)

[Aleksey Shipilev]

Bug:
  https://bugs.openjdk.java.net/browse/JDK-8211274

Fix:
  http://cr.openjdk.java.net/~shade/8211274/webrev.01/

This fixes the fatal build warnings when building on x86_32. New default to ShouldNotReachHere in
jniFastGetField_x86_32 is safe, because it matches the similar switch 30 lines above it.

Testing: x86_32 build, trivial sanity checks

Thanks,
-Aleksey