RFR: 8307147: [x86] Dangling pointer warning for Assembler::_attributes

[Kim Barrett]

Please review this change to work around a false positive -Wdangling-pointer
warning from gcc13.1.  The approach being taken is to suppress the warning,
with a comment describing why it's a false positive.  Also a little code
restructuring to make it more obvious.

I tried various code modifications to avoid the warning, but they were either
obscure, large and instrusive, or didn't seem reliably future-proof against
further changes in gcc's analysis.  And that's just for the attempts that
worked.

Testing:
mach5 tier1-3 with gcc11.2 (current default in Oracle's CI)

Local (linux-x64) tier1 with gcc13.1, and verified the relevant warnings are
not reported.  This required disabling compiler warnings as errors, as there
are other new warnings from gcc13.1: JDK-8307210 and JDK-8307196.

-------------

Commit messages:
 - suppress x86 InstructionAttr warning
 - warning disable pragma

Changes: https://git.openjdk.org/jdk/pull/13751/files
 Webrev: https://webrevs.openjdk.org/?repo=jdk&pr=13751&range=00
  Issue: https://bugs.openjdk.org/browse/JDK-8307147
  Stats: 29 lines in 4 files changed: 21 ins; 5 del; 3 mod
  Patch: https://git.openjdk.org/jdk/pull/13751.diff
  Fetch: git fetch https://git.openjdk.org/jdk.git pull/13751/head:pull/13751

PR: https://git.openjdk.org/jdk/pull/13751