Support using custom rustflags for targets

This is necessary for AVR and AMDGPU. We currently just hardcode some
flags in the binary, which is good enough. There's a new column which
keeps track of the used flags and shows them for a build.

A migration makes sure that the older results are properly backfilled
with the flags to make them build.

migrations/20250704160556_reset-missing-target-cpu.sql

@@ -0,0 +1,8 @@
+-- Add migration script here
+
+ALTER TABLE build_info
+    ADD COLUMN rustflags VARCHAR;
+
+DELETE FROM build_info WHERE target IN ('avr-none', 'amdgcn-amd-amdhsa') AND mode = 'core';
+
+DELETE FROM finished_nightly WHERE nightly > '2025-02-11';