etnaviv: add alternative NIR compiler

enable with ETNA_MESA_DEBUG=nir

Signed-off-by: Jonathan Marek <jonathan@marek.ca>
Reviewed-by: Philipp Zabel <p.zabel@pengutronix.de>
13 jobs for !1497 with etna-nir in 27 minutes and 41 seconds (queued for 8 seconds)
detached
Status Name Job ID Coverage
  Containers Build
passed debian #476928

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  Build+Test
passed meson-arm64 #476937

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passed meson-armhf #476936

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passed meson-clang #476930

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passed meson-clover #476934

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passed meson-i386 #476938

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passed meson-main #476933

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passed meson-swr-glvnd #476929

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passed meson-vulkan #476935

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passed scons-llvm #476940

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passed scons-nollvm #476939

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passed scons-swr #476931

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passed scons-win64 #476932

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