Initial support for compiler attribute statements

[noelchalmers]

Description

This added a generic ability of the OCCA parser to allow qualifier_t tokens to take arguments, and adds the __attribute__ keyword as a qualifier. Consequently, some initial __attribute__ and dllexport__ statements should be possible.

This is useful when using some backend-specific compiler attributes on variables/functions, e.g.

@kernel
__attribute__((amdgpu_waves_per_eu(1,1)))
void myKernel(...)

or when needing specific variable traits, e.g. with AMD GPUs and MFMA instructions:

for(int l=0;l<64;++l;@inner(0)){
  __attribute__((__vector_size__(4 * 8))) double C = {0.};
  
  ...

  // C = A*B
  C = __builtin_amdgcn_mfma_f64_16x16x4f64(A, B, C, 0, 0, 0);
}

[codecov]

Codecov Report

Merging #729 (4b9dd4d) into development (c17644b) will decrease coverage by 0.28%.
The diff coverage is 32.11%.

Additional details and impacted files

@@               Coverage Diff               @@
##           development     #729      +/-   ##
===============================================
- Coverage        75.24%   74.97%   -0.28%     
===============================================
  Files              298      298              
  Lines            19397    19487      +90     
===============================================
+ Hits             14595    14610      +15     
- Misses            4802     4877      +75     

Files	Coverage Δ
src/occa/internal/lang/keyword.cpp	82.19% <100.00%> (+0.18%)	⬆️
src/occa/internal/lang/modes/okl.cpp	93.67% <100.00%> (ø)
src/occa/internal/lang/qualifier.hpp	100.00% <ø> (ø)
src/occa/internal/lang/type/vartype.cpp	83.20% <0.00%> (-1.33%)	⬇️
src/occa/internal/lang/variable.cpp	69.76% <0.00%> (-1.67%)	⬇️
src/occa/internal/lang/loaders/typeLoader.cpp	71.55% <39.39%> (-7.45%)	⬇️
src/occa/internal/lang/qualifier.cpp	61.72% <29.23%> (-22.28%)	⬇️

... and 2 files with indirect coverage changes