Use cargo for lto over manually entering rustflags

.cargo/config

 [build]
-rustflags = ["-Cembed-bitcode=yes"]
+rustflags = [
+  # Kernel should preserve floating-point registers
+  "-Csoft-float",
+]

Cargo.toml

@@ -44,3 +44,13 @@ qemu_debug = []
 serial_debug = []
 system76_ec_debug = []
 slab = ["slab_allocator"]
+
+[profile.dev]
+# Kernel doesn't yet work great with debug mode :(
+opt-level = 3
+
+# LTO fixes some duplicate symbols of memcpy/memmove/etc
+lto = true
+
+[profile.release]
+lto = true