[clang] [Driver] Always use gas with -fno-integrated-as on Solaris (PR #65489)

[Fangrui Song via cfe-commits]

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+/// General tests for assembler invocations on Solaris.
+
+/// Test that clang uses gas on Solaris.
+// RUN: %clang -x assembler %s -### -c -fno-integrated-as \
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MaskRay wrote:

While here, consider testing `-Wa,` as well, e.g. `-Wa,--compress-debug-sections=zlib`

https://github.com/llvm/llvm-project/pull/65489