arch-gcn3,misc: Fix .fast compilation errors for GCN3_x86

Unused variable errors occurred when compiling gem5.fast with GCC. This
patch fixes this.

Change-Id: Iaca1fb8194c2381c0a4ba5d0ea1fb5b8f2a11829
Reviewed-on: https://gem5-review.googlesource.com/c/public/gem5/+/44885
Reviewed-by: Bobby R. Bruce <bbruce@ucdavis.edu>
Reviewed-by: Matt Sinclair <mattdsinclair@gmail.com>
Reviewed-by: Matthew Poremba <matthew.poremba@amd.com>
Maintainer: Bobby R. Bruce <bbruce@ucdavis.edu>
Tested-by: kokoro <noreply+kokoro@google.com>

src/dev/hsa/hsa_packet_processor.cc

@@ -133,8 +133,7 @@ HSAPacketProcessor::write(Packet *pkt)
           "%s: write of size %d to reg-offset %d (0x%x)\n",
           __FUNCTION__, pkt->getSize(), daddr, daddr);
 
-    int doorbellSize = gpu_device->driver()->doorbellSize();
-    assert(doorbellSize == pkt->getSize());
+    assert(gpu_device->driver()->doorbellSize() == pkt->getSize());
 
     uint64_t doorbell_reg(0);
     if (pkt->getSize() == 8)

src/gpu-compute/gpu_compute_driver.cc

@@ -582,7 +582,7 @@ GPUComputeDriver::ioctl(ThreadContext *tc, unsigned req, Addr ioc_buf)
 
             assert(isdGPU);
             assert((args->va_addr % TheISA::PageBytes) == 0);
-            Addr mmap_offset = 0;
+            M5_VAR_USED Addr mmap_offset = 0;
 
             Request::CacheCoherenceFlags mtype = defaultMtype;
             Addr pa_addr = 0;