spirv: work around libshaderc locale dependence
Do this by explicitly clearing LC_NUMERIC before calling into libshaderc. I'm not entirely sure whether libshaderc or glslang is to blame here, but either way, as long as it works - I'm happy. This is not that big of a deal performance-wise since shader compilation is infrequent, and it also doesn't need messy global initialization since we can just stuff the locale_t in with the struct priv.
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