chromium68: fix aarch64 link issue

chromium68: fix aarch64 link issue

Backport upstream patch:
https://chromium-review.googlesource.com/c/chromium/third_party/ffmpeg/+/1390286

Fix this error when linking libcbe.so:
  obj/third_party/ffmpeg/ffmpeg_internal/
    autorename_libavcodec_aarch64_h264idct_neon.o:

  relocation R_AARCH64_ADR_PREL_PG_HI21 against symbol
  `ff_h264_idct8_add_neon' which may bind externally can not be used
  when making a shared object; recompile with -fPIC

Also remove the mksnapshot path override for aarch64, it is not
necessary now we run mksnapshot with qemu. This fixes the install
step.

Bug-AGL: SPEC-2506

Change-Id: Idb90c8c20910ec48ed01f1b02528f14781edc4a3
Signed-off-by: Nick Yamane <nickdiego@igalia.com>
Signed-off-by: Jacobo Aragunde Pérez <jaragunde@igalia.com>