MIPS/GAS: Set default CPU to MIPS64r6 for 64-bit "img" configurations

Fix broken commit 070961b377b3 ("MIPS: Set r6 as default arch if vendor
is img") that sets up GAS in an inconsistent way where "img" vendor has
been used with a 64-bit configuration, such as `mips64-img-linux-gnu'.
In that case GAS is set up to use a 64-bit ABI by default combined with
the MIPS32r6 CPU, which is 32-bit.

Consequently GAS always fails to assemble even trivial input, producing
a message such as:

Assembler messages:
Error: -march=mips32r6 is not compatible with the selected ABI
.../gas/testsuite/gas/all/nop.s:2: Error: `gp=32' used with a 64-bit ABI

unless the defaults have been suitably overridden either for the ABI or
the CPU.

Set the default CPU to MIPS64r6 for 64-bit "img" vendor configurations
then and adjust the GAS testsuite accordingly, removing 1048 FAIL and 3
ERROR regression test results for the `mips64-img-linux-gnu' and
`mips64el-img-linux-gnu' targets each.