firmware/ksupport: Include .gcc_except_table (LSDA)

For whatever reason, no language-specific unwind data
was generated for ksupport code so far, but rustc does
emit it for an upcoming refactoring.

artiq/firmware/ksupport/ksupport.ld

@@ -60,6 +60,11 @@ SECTIONS
         KEEP(*(.eh_frame_hdr))
     } > ksupport :text :eh_frame
 
+    .gcc_except_table :
+    {
+        *(.gcc_except_table)
+    } > ksupport
+
     .data :
     {
         *(.data .data.*)