arm: warn/error on movrelx usage problematic with PIC on ELF

The warning has false positives but our asm does not trigger it. For
new code false positives can only be avoided by changing the register
allocation.

libavutil/arm/asm.S

@@ -184,6 +184,15 @@ T       ldr             \rd, [\rd]
 .endm
 
 .macro  movrelx         rd,  val, gp
+    .ifc \rd,\gp
+        .error      "movrelx needs two distinct registers"
+    .endif
+    .ifc \rd\()_\gp,r12_
+        .warning    "movrelx rd=\rd without explicit set gp"
+    .endif
+    .ifc \rd\()_\gp,ip_
+        .warning    "movrelx rd=\rd without explicit set gp"
+    .endif
 #if CONFIG_PIC && defined(__ELF__)
     .ifnb \gp
       .if .Lpic_gp