fastbootd: exporting CPU ABI info

CPU ABI info, e.g., arm64-v8a, can be useful to determine image
compatibility, prior to flash. Adding this info in fastbootd.

Bug: 74445765
Test: fastboot getvar cpu-abi # arm64-v8a
Change-Id: Ied494b646c551320295956b7890c0102fdb88382
diff --git a/fastboot/device/variables.h b/fastboot/device/variables.h
index 4dec10f..90840d6 100644
--- a/fastboot/device/variables.h
+++ b/fastboot/device/variables.h
@@ -63,6 +63,7 @@
                            std::string* message);
 bool GetSnapshotUpdateStatus(FastbootDevice* device, const std::vector<std::string>& args,
                              std::string* message);
+bool GetCpuAbi(FastbootDevice* device, const std::vector<std::string>& args, std::string* message);
 
 // Helpers for getvar all.
 std::vector<std::vector<std::string>> GetAllPartitionArgsWithSlot(FastbootDevice* device);