[Core] Changes from @amikstcyr to add native C compiler support

[dmed256]

Description

Work based on #306 (Thanks @amikstcyr 🎉)

• Adds additional properties to kernels:
  • Adds compiler_language option ('C' or 'C++' (default))
  • Changes linker_flags -> compiler_linker_flags for naming consistency
  • Adds compiler_shared_flags which can be passed with OCCA_COMPILER_SHARED_FLAGS environment variable
• Adds 12_native_c_kernels example
• Fixes failing tests
• Adds proper Intel OpenMP flag
• Fixes occa::array offsets

[codecov]

Codecov Report

❗ No coverage uploaded for pull request base (master@ac5a352). Click here to learn what that means.
The diff coverage is 61.29%.

@@            Coverage Diff            @@
##             master     #308   +/-   ##
=========================================
  Coverage          ?   70.92%           
=========================================
  Files             ?      282           
  Lines             ?    19470           
  Branches          ?        0           
=========================================
  Hits              ?    13810           
  Misses            ?     5660           
  Partials          ?        0

Impacted Files	Coverage Δ
src/lang/preprocessor.cpp	71.84% <ø> (ø)
include/occa/tools/sys.hpp	0% <ø> (ø)
src/core/launchedDevice.cpp	0% <0%> (ø)
src/lang/modes/withLauncher.cpp	0% <0%> (ø)
src/modes/openmp/utils.cpp	66.66% <0%> (ø)
bin/occa.cpp	38.46% <0%> (ø)
src/io/utils.cpp	99.4% <100%> (ø)
src/modes/openmp/device.cpp	88.09% <100%> (ø)
src/c/json.cpp	87.61% <100%> (ø)
include/occa/array/array.tpp	100% <100%> (ø)
... and 3 more