[MCJIT] Replace memcpy with readBytesUnaligned in RuntimeDyldMachOI386.

This should fix the failures of the MachO_i386_DynNoPIC_relocations.s test case
on MIPS hosts.
Add a new disassembly-format specification so that the disassembler
output style can be customized.  Change the built-in default to be
more similar to gdb's disassembly formatting.

The disassembly-format for a gdb-like output is

${addr-file-or-load} <${function.name-without-args}${function.concrete-only-addr-offset-no-padding}>:

The disassembly-format for the lldb style output is

{${function.initial-function}{${module.file.basename}`}{${function.name-without-args}}:\n}{${function.changed}\n{${module.file.basename}`}{${function.name-without-args}}:\n}{${current-pc-arrow} }{${addr-file-or-load}}:

The two backticks in the lldb style formatter triggers the sub-expression evaluation in
CommandInterpreter::PreprocessCommand() so you can't use that one as-is ... changing to
use ' characters instead of ` would work around that.

<rdar://problem/9885398>