x86/entry/vdso32: Omit '.cfi_offset eflags' for LLVM < 16

After commit:

  8849616 ("x86/entry/vdso32: Remove open-coded DWARF in sigreturn.S")

building arch/x86/entry/vdso/vdso32/sigreturn.S with LLVM 15 fails with:

  <instantiation>:18:20: error: invalid register name
   .cfi_offset eflags, 64
                     ^
  arch/x86/entry/vdso/vdso32/sigreturn.S:33:2: note: while in macro instantiation
   STARTPROC_SIGNAL_FRAME 8
   ^

Support for eflags as an argument to .cfi_offset was added in the LLVM
16 development cycle:

    llvm/llvm-project@67bd3c5 [1]

Only add this .cfi_offset directive if it is supported by the assembler
to clear up the error.

[ mingo: Tidied up the changelog and the comment a bit ]

Fixes: 8849616 ("x86/entry/vdso32: Remove open-coded DWARF in sigreturn.S")
Signed-off-by: Nathan Chancellor <nathan@kernel.org>
Signed-off-by: Ingo Molnar <mingo@kernel.org>
Acked-by: H. Peter Anvin (Intel) <hpa@zytor.com>
Link: https://patch.msgid.link/20260123-x86-vdso32-wa-llvm-15-cfi-offset-eflags-v1-1-0f412e3516a4@kernel.org

Author: nathanchance

arch/x86/entry/vdso/vdso32/sigreturn.S

@@ -22,7 +22,17 @@
 	CFI_OFFSET	cs,     IA32_SIGCONTEXT_cs
 	CFI_OFFSET	ss,     IA32_SIGCONTEXT_ss
 	CFI_OFFSET	ds,     IA32_SIGCONTEXT_ds
+/*
+ * .cfi_offset eflags requires LLVM 16 or newer:
+ *
+ *   https://github.com/llvm/llvm-project/commit/67bd3c58c0c7389e39c5a2f4d3b1a30459ccf5b7
+ *
+ * Check for 16.0.1 to ensure the support is present, as 16.0.0 may be a
+ * prerelease version.
+ */
+#if defined(CONFIG_AS_IS_GNU) || (defined(CONFIG_AS_IS_LLVM) && CONFIG_AS_VERSION >= 160001)
 	CFI_OFFSET	eflags, IA32_SIGCONTEXT_flags
+#endif
 .endm
 
 	.text