RFR: 8284578: Relax InterpreterCodelet stub alignment

[Aleksey Shipilev]

`InterpreterCodelet` is aligned by `CodeEntryAlignment` (`CAE`) twice. First, the entire stub is aligned, which aligns its data section. Then, the code section in the stub is aligned. Since `CAE` is usually larger than the size of `InterpreterCodelet`, we are wasting quite a bit of space for each codelet. In the extreme cases, like PPC that defaults to `CAE=128`, we have 16 bytes of codelet data effectively taking 128 bytes!

This can be made better by relaxing the `InterpreterCodelet` stub alignment to `HeapWordSize`, while leaving its code section alignment the same.

This tangentially touches the only other user for `StubQueue`: `ICStub`. Unfortunately, we cannot do the same kind of relaxation there, because there is a reverse lookup function that needs to reach data section from the code section, which forces us to keep the same alignment for both.

Interpreter sizes on Linux x86_64 release:


# Baseline, CEA=32 (default)
code size        =     94K bytes
avg codelet size =    356  bytes

# Baseline, CEA=128 (PPC-like)
code size        =    133K bytes
avg codelet size =    501  bytes

# Patched, CEA=32 (default)
code size        =     89K bytes
avg codelet size =    338  bytes

# Patched, CEA=128 (PPC-like)
code size        =    100K bytes
avg codelet size =    380  bytes


Point performance runs (SPECjvm2008:serial with `-Xint` on Linux x86_64 release):


Benchmark     Mode  Cnt   Score   Error  Units

# Baseline, CEA=32
Serial.test  thrpt    9  73.427 ± 0.152  ops/s

# Baseline, CEA=128
Serial.test  thrpt    9  70.999 ± 0.246  ops/s

# Patched, CEA=32
Serial.test  thrpt    9  73.991 ± 0.860  ops/s

# Patched, CEA=128
Serial.test  thrpt    9  72.981 ± 0.301  ops/s


Additional testing:
 - [x] Linux x86_64 fastdebug `tier1`
 - [ ] Linux x86_64 fastdebug `tier2`
 - [ ] Linux x86_64 fastdebug `tier3`

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Commit messages:
 - Initial implementation

Changes: https://git.openjdk.java.net/jdk/pull/8159/files
 Webrev: https://webrevs.openjdk.java.net/?repo=jdk&pr=8159&range=00
  Issue: https://bugs.openjdk.java.net/browse/JDK-8284578
  Stats: 43 lines in 4 files changed: 16 ins; 5 del; 22 mod
  Patch: https://git.openjdk.java.net/jdk/pull/8159.diff
  Fetch: git fetch https://git.openjdk.java.net/jdk pull/8159/head:pull/8159

PR: https://git.openjdk.java.net/jdk/pull/8159