Make gdb.arch/i386-biarch-core.exp more robust

This test attempts to load a x86 core file no matter what target
architectures the tested GDB supports. If GDB doesn't know how to handle
a i386 target, it is very likely the core file will not be recognized.

In this case we should still attempt to load a core file to make sure GDB
doesn't crash or throws an internal error.  But we should not proceed to
try to read memory unconditionally.

This patch makes the test check for proper i386 arch support in GDB and bails
out if i386 is not supported and the core file format is not recognized.

This addresses the spurious aarch64-elf failures i'm seeing for this test.

gdb/testsuite/ChangeLog:

2017-02-13  Luis Machado  <lgustavo@codesourcery.com>

	* gdb.arch/i386-biarch-core.exp: Check for i386 arch support and
	return if core file is not recognized.