Fix nacl-gdb so that it can read ELF core dump files

Before this change, nacl-gdb gives the error "no core file handler
recognizes format" when given an ELF code dump file.

Fix this by setting up the register layout for core dumps.  Reuse the
same layout as for Linux code dumps for interoperability.

I tested this by producing a minidump from a NaCl minidump generator
and converting it to an ELF core dump using Breakpad's minidump-2-core
tool.  While we could generate a different register layout (e.g. using
NaCl's "struct NaClExceptionContext"), it's useful to keep the core
dumps readable by regular Linux GDB.

TEST=see above

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5 files changed