Kernel: Add sys$set_coredump_metadata() syscall

This can be used by applications to store information (key/value pairs)
likely useful for debugging, which will then be embedded in the coredump.

Kernel/API/Syscall.h

@@ -196,7 +196,8 @@ namespace Kernel {
     S(adjtime)                \
     S(allocate_tls)           \
     S(prctl)                  \
-    S(mremap)
+    S(mremap)                 \
+    S(set_coredump_metadata)
 
 namespace Syscall {
 
@@ -442,6 +443,11 @@ struct SC_ptrace_peek_params {
     u32* out_data;
 };
 
+struct SC_set_coredump_metadata_params {
+    StringArgument key;
+    StringArgument value;
+};
+
 void initialize();
 int sync();