RFR: 8357119: Make Zero the default and only variant of HotSpot for iOS

[Julian Waters]

Currently, the Zero variant of HotSpot must be manually selected for mobile to compile properly for iOS, since iOS does not allow writeable and executable sections. Since Zero is, to my knowledge, the only valid HotSpot variant for iOS, this default should be handled by the build rather than require the developer to have specific knowledge about iOS and the mobile fork. I also took the opportunity to explicitly disallow selecting other JVM variants at all for iOS, this can be rolled back if my assumption turns out to be incorrect. While here, also touch up the build docs a little.

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Commit messages:
 - Make Zero the only valid JVM for hotspot.m4

Changes: https://git.openjdk.org/mobile/pull/35/files
  Webrev: https://webrevs.openjdk.org/?repo=mobile&pr=35&range=00
  Issue: https://bugs.openjdk.org/browse/JDK-8357119
  Stats: 9 lines in 1 file changed: 8 ins; 0 del; 1 mod
  Patch: https://git.openjdk.org/mobile/pull/35.diff
  Fetch: git fetch https://git.openjdk.org/mobile.git pull/35/head:pull/35

PR: https://git.openjdk.org/mobile/pull/35