Kernel+SystemServer: Don't hardcode coredump directory path

Instead, allow userspace to decide on the coredump directory path. By
default, SystemServer sets it to the /tmp/coredump directory, but users
can now change this by writing a new path to the sysfs node at
/sys/kernel/variables/coredump_directory, and also to read this node to
check where coredumps are currently generated at.

Kernel/Coredump.cpp

@@ -9,6 +9,7 @@
 
 #include <AK/ByteBuffer.h>
 #include <AK/JsonObjectSerializer.h>
+#include <AK/Singleton.h>
 #include <Kernel/Coredump.h>
 #include <Kernel/FileSystem/Custody.h>
 #include <Kernel/FileSystem/OpenFileDescription.h>
@@ -22,8 +23,15 @@
 
 #define INCLUDE_USERSPACE_HEAP_MEMORY_IN_COREDUMPS 0
 
+static Singleton<SpinlockProtected<OwnPtr<KString>>> s_coredump_directory_path;
+
 namespace Kernel {
 
+SpinlockProtected<OwnPtr<KString>>& Coredump::directory_path()
+{
+    return s_coredump_directory_path;
+}
+
 bool Coredump::FlatRegionData::looks_like_userspace_heap_region() const
 {
     return name().starts_with("LibJS:"sv) || name().starts_with("malloc:"sv);