Avoid self-test failures on x86-linux

Avoid self-test failures on x86-linux

The disassembly tests in "maint selftest" will fail on x86-linux.
This happens because opcodes rejects an attempt to disassemble for an
arch with a 64-bit address size when bfd_vma is 32-bit.

This patch avoids this problem by avoiding the test in this case.  I
chose to do it this way because this seems to be the only situation
where opcodes checks the size of bfd_vma.

For v2 of this patch, I've also updated memory_error_test to do the
same thing.  This is needed due to the "improve error reporting from
the disassembler" patch.