[AMDGPU] Lazily init pal metadata on first function

Delay reading global metadata until the first function or the end of
the file is emitted. That way, earlier module passes can set metadata
that is emitted in the ELF.

`emitStartOfAsmFile` gets called when the passes are initialized,
which prevented earlier passes from changing the metadata.

This fixes issues encountered after converting
AMDGPUResourceUsageAnalysis to a Module pass in D117504.

Differential Revision: https://reviews.llvm.org/D118492