platform: Change detection of device code for OpenMP

[stotko]

For the OpenMP backend, the platform macros currently always report code to be STDGPU_CODE_HOST. Since some STDGPU_HOST_DEVICE annotated function may use different paths for host and device code and the device path may contain backend-specific optimizations, this choice will lead to backend-specific code being ignored.

Change the detection code to always report STDGPU_CODE_DEVICE rather than STDGPU_CODE_HOST. Note that for such functions using different code paths, these paths should result in equivalent behavior anyways.

[codecov]

Codecov Report

Merging #174 into master will not change coverage.
The diff coverage is n/a.

@@           Coverage Diff           @@
##           master     #174   +/-   ##
=======================================
  Coverage   97.80%   97.80%           
=======================================
  Files          28       28           
  Lines        1825     1825           
=======================================
  Hits         1785     1785           
  Misses         40       40           

Impacted Files	Coverage Δ
src/stdgpu/impl/atomic_detail.cuh	98.44% <0.00%> (ø)

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