[AArch64][compiler-rt] Disable SME ABI routines if function multiversioning is not supported (#94973)

This prevents build failures when building with `DISABLE_AARCH64_FMV`,
see #92921.

Author: kmclaughlin-arm

compiler-rt/lib/builtins/CMakeLists.txt

@@ -561,12 +561,23 @@ set(aarch64_SOURCES
   aarch64/fp_mode.c
 )
 
-if(COMPILER_RT_HAS_AARCH64_SME AND COMPILER_RT_HAS_FNO_BUILTIN_FLAG AND (COMPILER_RT_HAS_AUXV OR COMPILER_RT_BAREMETAL_BUILD))
-  list(APPEND aarch64_SOURCES aarch64/sme-abi.S aarch64/sme-abi-init.c aarch64/sme-abi-vg.c aarch64/sme-libc-routines.c)
-  message(STATUS "AArch64 SME ABI routines enabled")
-  set_source_files_properties(aarch64/sme-libc-routines.c PROPERTIES COMPILE_FLAGS "-fno-builtin")
-else()
-  message(STATUS "AArch64 SME ABI routines disabled")
+if (COMPILER_RT_HAS_AARCH64_SME)
+  if (NOT COMPILER_RT_DISABLE_AARCH64_FMV AND COMPILER_RT_HAS_FNO_BUILTIN_FLAG AND (COMPILER_RT_HAS_AUXV OR COMPILER_RT_BAREMETAL_BUILD))
+    list(APPEND aarch64_SOURCES aarch64/sme-abi.S aarch64/sme-abi-init.c aarch64/sme-abi-vg.c aarch64/sme-libc-routines.c)
+    message(STATUS "AArch64 SME ABI routines enabled")
+    set_source_files_properties(aarch64/sme-libc-routines.c PROPERTIES COMPILE_FLAGS "-fno-builtin")
+  else()
+    if(COMPILER_RT_DISABLE_AARCH64_FMV)
+      message(WARNING "AArch64 SME ABI routines require function multiversioning support.")
+    endif()
+    if(NOT COMPILER_RT_HAS_FNO_BUILTIN_FLAG)
+      message(WARNING "AArch64 SME ABI routines require '-fno-builtin'")
+    endif()
+    if(NOT (COMPILER_RT_HAS_AUXV OR COMPILER_RT_BAREMETAL_BUILD))
+      message(WARNING "AArch64 SME ABI routines requires sys/auxv.h or COMPILER_RT_BAREMETAL_BUILD flag")
+    endif()
+    message(STATUS "AArch64 SME ABI routines disabled")
+  endif()
 endif()
 
 # Generate outline atomics helpers from lse.S base