RFR: 8365312: GCC 12 cannot compile SVE on aarch64 with auto-var-init pattern

[Fei Gao]

`-ftrivial-auto-var-init=pattern` [1] supported by gcc-12 was added recently to help detect uses of uninitialized memory. [2] Under this extension mode, `__builtin_clear_padding` is called to clear the padding bits in object representation. But the builtin function does not support variable length aggregates [3].

`vfloat2_sve_sleef` is a typedef of `svfloat32x2_t`, which is defined as an opaque sizeless type in ARM C Language Extensions (ACLE) [4]. When `__builtin_clear_padding` is applied to such a type, it triggers the unsupported code path and results in build failures when compiling libsleef on Linux-AArch64, as reported in JDK-8364185 [5].

Switching the initialization mode with gcc-12 from `pattern` to `zero` avoids the use of this unsupported `__builtin_clear_padding` and resolves the issue.

I did not encounter similar problems when building with Clang-16 [6].

This patch changes the initialization mode from `pattern` to `zero` when compiling libsleef SVE component on Linux-AArch64 with gcc-12 to fix the build failure and help avoid potential security issues related to uninitialized padding

Testing (combined with https://github.com/openjdk/jdk/pull/26577):
1. Release and fastdebug build with gcc-11, gcc-12, and clang-16
2. `test/jdk/jdk/incubator/vector` passed on both 256-bit SVE and NEON machines

[1] https://gcc.gnu.org/onlinedocs/gcc-12.1.0/gcc/Optimize-Options.html#index-ftrivial-auto-var-init
[2] https://github.com/openjdk/jdk/commit/fae37aaae8b36fd74309b84fa1fdf017c7d932ed
[3] https://gcc.gnu.org/pipermail/gcc-patches/2020-November/559214.html
[4] https://arm-software.github.io/acle/main/acle.html#arm_sveh
[5] https://bugs.openjdk.org/browse/JDK-8364185
[6] https://releases.llvm.org/16.0.0/tools/clang/docs/ReleaseNotes.html

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Commit messages:
 - 8365312: __builtin_clear_padding not supported for variable length aggregates

Changes: https://git.openjdk.org/jdk/pull/26755/files
  Webrev: https://webrevs.openjdk.org/?repo=jdk&pr=26755&range=00
  Issue: https://bugs.openjdk.org/browse/JDK-8365312
  Stats: 11 lines in 1 file changed: 11 ins; 0 del; 0 mod
  Patch: https://git.openjdk.org/jdk/pull/26755.diff
  Fetch: git fetch https://git.openjdk.org/jdk.git pull/26755/head:pull/26755

PR: https://git.openjdk.org/jdk/pull/26755