add jl_get_cpu_name function to libjulia

thin ccall'able wrapper around llvm::sys::getHostCPUName
Modify jl_get_cpu_name to use jl_pchar_to_string,

add Sys.cpu_name
Move cpu_name into init_sysinfo()
Demote error-demonstrating doctest block

This is so that make doctest will pass again if you are not using a user account called sabae on a Mac.
Better documentation for gcdx
Include file information in emitted debug info
stdlib/base docs: convert examples to doctests

The standard library documentation contains many examples which can be processed as doctests.

Also includes formatting fixes:
- correctly wrap code references in double backquote environments
- Better cross-referencing of functions

Additionally, include some content fixes:

- A better description of gcdx
- Fixed embarrassing errors in iseven and isodd documentation. Contrary to the previous version of the docs, claimed that 9 is not even and 10 is not odd.
X.Y.Z-dev denotes open to breaking changes

X.Y.Z-pre now denotes prerelease planning, e.g. soft feature freeze.
Header improvements to conform to MELPA's standards.
In backtraces, the scope may now not be the DISubprogram
All julia functions and variables should be prefixed.

Elisp doesn't have namespaces, so this is the best way to prevent name
clashes between packages. However, it's possible that users may have
Emacs configuration that calls the functions with their old names, so
I've set up aliases.
Merge pull request #7979 from Wilfred/fixes_for_melpa

Fixes for MELPA
Merge pull request #6660 from JuliaLang/kf/filediinfo

Include file information in emitted debug info
Two small llvmcall fixes imported from cxx branch
Merge pull request #7552 from tkelman/getcpuname

RFC: add jl_get_cpu_name function to libjulia
doc: update helpdb.jl
Add --dump-bitcode option to dump bitcode when building the system image
Add RPATH_ORIGIN analogue to OSX as well
WIP: Add ability to build a custom libcxx

This is useful when compiling on a system with an outdated std c++ library,
but you need to build llvm, or when wanting to build julia with msan.
Also get libcxx with make get-llvm