Fix ill-formed 'inline inline' in the ARM __spin_loop_pause

[bjornpagen]

Commit f0e8ae6 ("Create a stdexec module", #2138) changed the ARM branch of __spin_loop_pause from static to inline. This change made the definition safe to include in the module purview. After this change, the declaration reads:

STDEXEC_ATTRIBUTE(always_inline) inlinevoid__spin_loop_pause() noexcept

On GCC and Clang, STDEXEC_ATTRIBUTE(always_inline) already ends in inline. The macro STDEXEC_ATTRIBUTE_CASE_ALWAYS_INLINE expands to __attribute__((__always_inline__, __artificial__)) inline. Clang adds __nodebug__. So the declaration contains inline inline. This is ill-formed. See [dcl.spec.general]/2: "At most one of each of the decl-specifiers friend, typedef, or inline shall appear in a decl-specifier-seq."

Both compilers give a diagnostic, as the standard requires. GCC rejects the code with a hard error:

include/stdexec/__detail/__spin_loop_pause.hpp:42:36: error: duplicate 'inline'

This error breaks every GCC build on arm/aarch64 since that commit. Clang accepts the code with a warning (-Wduplicate-decl-specifier, on by default). CI stayed green because no CI leg compiles the ARM branch. The Linux and Windows runners are x86_64. The macOS jobs use macos-26-large runners, which are Intel.

This PR makes two changes. Together they make ALWAYS_INLINE guarantee inline linkage on every compiler:

1. Remove the explicit inline at the ARM use site. The macro supplies inline on GCC and Clang. On MSVC, __forceinline implies it. The HEAD spelling __forceinline inline likely causes warning C4141 there as well.
2. Add a plain inline to the fallback branch of STDEXEC_ATTRIBUTE_CASE_ALWAYS_INLINE. nvcc, NVHPC, and other EDG front ends take the fallback branch, because they define neither STDEXEC_CLANG() nor STDEXEC_GCC(). Before this PR, the macro expanded to nothing for them. The explicit inline at the ARM site was the only thing that kept this header-defined function ODR-safe for them. The fallback inline keeps that property. (I verified this by reading the detection chain in __config.hpp. I did not run nvcc or NVHPC.)

Site survey: the attribute has 81 uses. The ARM site was the only STDEXEC_ATTRIBUTE(always_inline) inline pairing. Two sites pair the attribute with static (the x86 and fallback-architecture branches of this same function). These become inline static on GCC and Clang. That is well-formed and keeps internal linkage.

Verification, on aarch64-apple-darwin:

• GCC 16.1 (-std=c++26, with and without -fno-exceptions) and Clang 22 compile the header cleanly after this change.
• At HEAD, GCC errors and Clang warns.
• With this fix, the full default test suite builds and passes with GCC 16.1 on this host: 963/968. The 5 failures are the relacy binaries. They fail identically with and without this change, so they are environmental on this host. At HEAD, they do not build at all.
• The -fno-exceptions test configuration from the CI matrix also passes: 879/879.

Note: the x86 and fallback-architecture branches still spell static. If the module-purview concern from #2138 applies to them, they may need the same treatment. I did not touch them, to keep this a pure build fix. I can also send a separate PR that adds an arm64 macOS CI leg (macos-26-class runners are Apple Silicon), so CI catches this class of break.

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