Add more circleci build for testing in x86_64 env

[nomaddo]

Related: #39

This pullreq add more circleci build specially for building x86_64 versions of the compiler for testing.
This pullreq MUST accept after this pullreq:
doe300/VC4CLStdLib#4

I added two jobs:

• Build VC4C using SPIRV-LLVM
• Build VC4C using llvm-lib

The purpose of this pullreq is comparison between one of SPIRV-LLVM and LLVM-LIB with ease.

[doe300]

Assuming the native builds compile and run (which I haven't tested), this is a great improvement, thanks!
See my comment for some minor changes.

[doe300]

TestVC4C runs about 15 mins (on my desktop PC), so running it twice on every build is not an option.
I selected a few test-cases which run quite fast (and am working on adding some more):

build/test/TestVC4C --output=plain --mode=verbose --test-instructions --test-operators

Same for the test beneath

[nomaddo]

Fixed.

BTW: Compiling with SPIRV-LLVM is a little bit longer than llvm's.
Is it still better front-end than others?

[doe300]

SPIRV-Tools takes a long time to compile (a few minutes).

While compiling OpenCL-kernels, using the SPIRV-LLVM is faster than using default clang, at least in my experience. Or do you have other measurements?

Edit:
On the question, whether the SPIR-V front-end is "better" than the LLVM library front-end? Maybe at experimental support for image types/operations, but other than that, no.