Add kernel cpu=false and context accessor

[vchuravy]

Addresses the need in #377.

[codecov]

Codecov Report

Patch coverage: 73.21% and project coverage change: -0.12 ⚠️

Comparison is base (a07ec99) 87.56% compared to head (2af3018) 87.45%.

Additional details and impacted files

@@            Coverage Diff             @@
##             main     #389      +/-   ##
==========================================
- Coverage   87.56%   87.45%   -0.12%     
==========================================
  Files           8        8              
  Lines         539      582      +43     
==========================================
+ Hits          472      509      +37     
- Misses         67       73       +6     

Impacted Files	Coverage Δ
src/KernelAbstractions.jl	80.48% <53.33%> (-2.63%)	⬇️
src/cpu.jl	80.76% <70.37%> (-1.59%)	⬇️
src/macros.jl	98.80% <100.00%> (+0.68%)	⬆️

... and 2 files with indirect coverage changes

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