Split x86 arch into i386 and x86_64

[gctucker]

We currently only have one x86 arch called x86. Depending on the defconfig, the resulting binary will be 32 or 64-bit (i386_defconfig or x86_64_defconfig respectively). While this is fine from a kernel point of view, it makes it impossible to differentiate the user-space file system to pick. So all x86 tests are currently all running with i386 user-space since it's compatible with both architectures.

To solve this issue, we should remove the x86 in our list of architectures and use i386 and x86_64 instead. We would then only build respective defconfigs for each arch, which can be set using filters in the test configs. We can also then map the kernel architectures with Buildroot and Debian file systems. For example, for Debian, we currently have this (because we call the 32-bit file system x86):

    arch_map:
      armhf: {arch: arm}

We should start calling the x86 file systems following the Debian standard architecture names, i.e. i386 and amd64. This would then give us the following mapping:

    arch_map:
      armhf: {arch: arm}
      amd64: {arch: x86_64}

Both the kernel and Debian use i386, and we won't have x86 any more, so this should be all that's needed there.

If we just rename the Debian file systems and don't update the list of kernel architectures, we'll have this interim mapping:

    arch_map:
      armhf: {arch: arm}
      x86: {arch: i386}

[khilman]

Let's go for the rename. We only have a few x86 platforms as it is, so now is the time to clean it up.