[release/6.0] Define __cpuid{ex} only when there's no builtin one (backport of #73065)

[ayakael]

Customer impact

Without this, build with clang15 fails.

Testing

• unit tests in ci
• source-build build within Alpine environment passes

Risk

Low, as it is already in main, and only activates when these variables aren't defined

---

Backport of #73065 and #73905

Per @am11:

Fix clang 15 RC1 build: error: definition of builtin function '__cpuid'.
Also add clang 15 autodetection; upstream PR https://github.com/dotnet/arcade/pull/10199.

* gcc < v10 does not define the __has_builtin macro which I missed in unixstubs.cpp when making the previous change: https://github.com/dotnet/runtime/commit/992cf8c97cc71d4ca9a0a11e6604a6716ed4cefc. Some community legs like illumos use gcc v8.4 which were failing.
* gcc v10+ supports __has_builtin macro which was being skipped due to version based conditions and we were forcing the fallback code path.
This PR moves __has_builtin macro check in two root headers which cover all the sources that are using it (pal.h and gcenv.base.h)

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Issue Details

---

Backport of #73065

Per @am11:

Fix clang 15 RC1 build: error: definition of builtin function '__cpuid'.
Also add clang 15 autodetection; upstream PR https://github.com/dotnet/arcade/pull/10199.

Author:	ayakael
Assignees:	-
Labels:	area-Infrastructure-libraries
Milestone:	-

[am11]

There was a problem in this patch with old gcc versions. It was fixed in #73905.

[carlossanlop]

@jkotas I see you approved the main PR. When/if this is ready, can you please add the servicing-consider label and send an email to Tactics requesting merge approval?

[ghost]

Tagging subscribers to this area: @hoyosjs
See info in area-owners.md if you want to be subscribed.

Issue Details

---

Backport of #73065 and #73905

Per @am11:

Fix clang 15 RC1 build: error: definition of builtin function '__cpuid'.
Also add clang 15 autodetection; upstream PR https://github.com/dotnet/arcade/pull/10199.

* gcc < v10 does not define the __has_builtin macro which I missed in unixstubs.cpp when making the previous change: https://github.com/dotnet/runtime/commit/992cf8c97cc71d4ca9a0a11e6604a6716ed4cefc. Some community legs like illumos use gcc v8.4 which were failing.
* gcc v10+ supports __has_builtin macro which was being skipped due to version based conditions and we were forcing the fallback code path.
This PR moves __has_builtin macro check in two root headers which cover all the sources that are using it (pal.h and gcenv.base.h)

Customer impact

Without this, build with clang15 fails.

Testing

• unit tests in ci
• source-build build within Alpine environment passes

Risk

Low, as it is already in main, and only activates when these variables aren't defined

Author:	ayakael
Assignees:	-
Labels:	area-Infrastructure-libraries, area-Infrastructure-coreclr, community-contribution
Milestone:	6.0.x

[jeffschwMSFT]

approved. we will take for consideration in 6.0.x

[carlossanlop]

The CI failure is weird, because the test is supposed to be disabled (it was disabled here):

runtime/src/libraries/System.Diagnostics.EventLog/tests/System/Diagnostics/Reader/ProviderMetadataTests.cs

Line 94 in 2a65e07

[ActiveIssue("https://github.com/dotnet/runtime/issues/64153")]

Aside from that, the CI is green, it was approved by Tactics, signed off by area owner, and does not need OOB package authoring changes (it's native code). Ready to merge.

[carlossanlop]

The CI failure is weird, because the test is supposed to be disabled (it was disabled here):

Ah, this is 6.0, I thought this was 7.0, that's why we're hitting it here too. I'll backport the ActiveIssue change.