arm: Only output eabi attributes if building for ELF

This matches the other eabi attribute in the same file. This is
required in order to build for arm/hardfloat with other object
file formats than ELF.
Signed-off-by: Martin Storsjö <martin@martin.st>

libavutil/arm/asm.S

@@ -291,7 +291,7 @@ T       sub             \rn, \rn, \rm
 .endm
 
 #if HAVE_VFP_ARGS
-        .eabi_attribute 28, 1
+ELF     .eabi_attribute 28, 1
 #   define VFP
 #   define NOVFP @
 #else