android: enable assembly optimizations for x86_64

According to https://trac.ffmpeg.org/ticket/4928

 only x86 was affected
by the text relocation bug, not x86_64.

Change-Id: I1f4edbdbff53a2d19908cafb70d8307db069c0ca
Reviewed-by: Guillaume Roguez <guillaume.roguez@savoirfairelinux.com>

contrib/src/ffmpeg/rules.mak

@@ -106,9 +106,6 @@ FFMPEGCONF += \
 ifeq ($(ARCH),i386)
 FFMPEGCONF += --disable-asm
 endif
-ifeq ($(ARCH),x86_64)
-FFMPEGCONF += --disable-asm
-endif
 else
 # Desktop Linux
 FFMPEGCONF += \