softpipe: Implement ATOMFADD and enable cap TGSI_ATOMFADD
This enables the following piglits with PASS: nv_shader_atomic_float/execution/ shared-atomicadd-float shared-atomicexchange-float ssbo-atomicadd-float ssbo-atomicexchange-float
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This enables the following piglits with PASS: nv_shader_atomic_float/execution/ shared-atomicadd-float shared-atomicexchange-float ssbo-atomicadd-float ssbo-atomicexchange-float