RFR: Drop x86_32 code from x86_64-specific files

[Aleksey Shipilev]

There are instances of x86_32 code in x86_64-specific files. That code would not be ever used and thus would bit rot, as it would never be compiled on x86_32. The porting to x86_32 would make the x86_32-specific versions of the StubGenerator. This simplifies x86_64 StubGenerator a bit.

Additional testing:
 - [x] Linux x86_64 fastdebug `jdk_loom hotspot_loom`

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Commit messages:
 - Reinstate r15_thread reinitialization
 - Drop x86_32 code from x86_64-specific files

Changes: https://git.openjdk.java.net/loom/pull/77/files
 Webrev: https://webrevs.openjdk.java.net/?repo=loom&pr=77&range=00
  Stats: 32 lines in 1 file changed: 0 ins; 16 del; 16 mod
  Patch: https://git.openjdk.java.net/loom/pull/77.diff
  Fetch: git fetch https://git.openjdk.java.net/loom pull/77/head:pull/77

PR: https://git.openjdk.java.net/loom/pull/77