targets/swan: rename group build tag stm32l4x5 => stm32l4y5 to avoid clash with stm32 device name

Previously, there was no specific stm32f4r5.svd in lib/stm32-svd,
just stm32f4x5.svd was used; now, both files are present. This means
that the existing build-tag stm32f4r5 will include the
device/stm32/stm32f4r5.go file, and the additional build-tag stm32f4x5
would include the device/stm32/stm32f4x5.go file as well, resulting
in build conflicts. Renaming just the tag, which is used in src/machine,
and src/runtime, into stm32f4y5 solves this issue.

src/machine/machine_stm32l4x5.go

@@ -1,4 +1,4 @@
-//go:build stm32l4x5
+//go:build stm32l4y5
 
 package machine
 

src/runtime/runtime_stm32l4x5.go

@@ -1,4 +1,4 @@
-//go:build stm32 && stm32l4x5
+//go:build stm32 && stm32l4y5
 
 package runtime
 

targets/swan.json

@@ -1,6 +1,6 @@
 {
     "inherits": ["cortex-m4"],
-    "build-tags": ["swan", "stm32l4r5", "stm32l4x5", "stm32l4", "stm32"],
+    "build-tags": ["swan", "stm32l4r5", "stm32l4y5", "stm32l4", "stm32"],
     "serial": "uart",
     "linkerscript": "targets/stm32l4x5.ld",
     "extra-files": [