Update one integration test to use the cfg(target_arch = "x86_64") syntax

Author: phil-opp

Diff for: tests/test_kernels/map_phys_mem/Cargo.toml

@@ -4,7 +4,7 @@ version = "0.1.0"
 authors = ["Philipp Oppermann <[email protected]>"]
 edition = "2018"
 
-[dependencies]
+[target.'cfg(target_arch = "x86_64")'.dependencies]
 bootloader = { path = "../../.." }
 x86_64 = { version = "0.13.2", default-features = false, features = ["instructions", "inline_asm"] }
 uart_16550 = "0.2.10"