RFR: 8269122: The use of "extern const" for Register definitions generates poor code

[Andrew Haley]

Register definitions in HotSpot are declared as "extern const"
for ancient-historical reasons. We should stop doing that: it would
make the assembler significantly faster and smaller, improving both
bootstrap time and compilation speed.

This change shaves 2% off the size of the text section of libjvm.so on AArch64.

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Commit messages:
 - 8269122: The use of "extern const" for Register definitions generates poor code

Changes: https://git.openjdk.java.net/jdk/pull/4554/files
 Webrev: https://webrevs.openjdk.java.net/?repo=jdk&pr=4554&range=00
  Issue: https://bugs.openjdk.java.net/browse/JDK-8269122
  Stats: 24 lines in 1 file changed: 22 ins; 2 del; 0 mod
  Patch: https://git.openjdk.java.net/jdk/pull/4554.diff
  Fetch: git fetch https://git.openjdk.java.net/jdk pull/4554/head:pull/4554

PR: https://git.openjdk.java.net/jdk/pull/4554