gpu_mem: enable BTF

Generate BTF debug info when building gpu_mem.o. bpfloader will load
map info alongside gpu_mem_total_map, which allows pretty-printing via
the "cat" command for easier debugging.

Test: m gpu_mem.o
Test: gpuservice_unittest --gtest_filter=GpuMem*
Test: cat /sys/fs/bpf/map_gpu_mem_gpu_mem_total_map
Bug: 203823368
Signed-off-by: Connor O'Brien <connoro@google.com>
Change-Id: I453766c0f76e1aba5720400a0ec2e2f871967a91