qemu: bump to 2.12.0

qemu: bump to 2.12.0

In addition:
  - Update 0001-user-exec-fix-usage-of-mcontext-structure-on-ARM-uCl.patch
    with new line numbers and file location.
  - Remove upstream 0002-memfd-fix-configure-test.patch
  - Add new options found in 2.12.0 in qemu.mk as disabled.
  - Remove --with-system-pixman as it's no longer optional.

Tested with test-pkg:

./utils/test-pkg -p qemu -c configs/qemu_min_defconfig
                             br-arm-full [1/6]: OK
                  br-arm-cortex-a9-glibc [2/6]: OK
                   br-arm-cortex-m4-full [3/6]: SKIPPED
                          br-x86-64-musl [4/6]: OK
                      br-arm-full-static [5/6]: OK
                armv5-ctng-linux-gnueabi [6/6]: OK
6 builds, 1 skipped, 0 build failed, 0 legal-info failed

Signed-off-by: Adam Duskett <aduskett@gmail.com>
Signed-off-by: Thomas Petazzoni <thomas.petazzoni@bootlin.com>