arm: Suppress tags about used cpu arch and extensions

When all the codepaths using manually set .arch/.fpu code is
behind runtime detection, the elf attributes should be suppressed.

This allows tools to know that the final built binary doesn't
strictly require these extensions.

Signed-off-by: Martin Storsjö <martin@martin.st>
(cherry picked from commit dcae2e32f7
and b77e335e44)
Signed-off-by: Martin Storsjö <martin@martin.st>

configure

@@ -1204,6 +1204,7 @@ HAVE_LIST="
     alsa_asoundlib_h
     altivec_h
     arpa_inet_h
+    as_object_arch
     asm_mod_q
     asm_mod_y
     attribute_may_alias
@@ -3223,6 +3224,11 @@ EOF
 
     enabled_all armv6t2 shared !pic && enable_pic
 
+    # llvm's integrated assembler supports .object_arch from llvm 3.5
+    [ "$objformat" = elf ] && check_as <<EOF && enable as_object_arch
+.object_arch armv4
+EOF
+
 elif enabled mips; then
 
     check_inline_asm loongson '"dmult.g $1, $2, $3"'

libavutil/arm/asm.S

@@ -43,11 +43,17 @@
 #elif HAVE_ARMV5TE
         .arch           armv5te
 #endif
+#if   HAVE_AS_OBJECT_ARCH
+ELF     .object_arch    armv4
+#endif
 
 #if   HAVE_NEON
         .fpu            neon
+ELF     .eabi_attribute 10, 0           @ suppress Tag_FP_arch
+ELF     .eabi_attribute 12, 0           @ suppress Tag_Advanced_SIMD_arch
 #elif HAVE_VFP
         .fpu            vfp
+ELF     .eabi_attribute 10, 0           @ suppress Tag_FP_arch
 #endif
 
         .syntax unified