LibX86: Add an X86 instruction decoder library + basic disassembler

This will be very useful for developer tools like ProfileView, and also
for future tools like debuggers and such. :^)

Userland/disasm.cpp

@@ -0,0 +1,27 @@
+#include <AK/LogStream.h>
+#include <AK/MappedFile.h>
+#include <LibX86/Disassembler.h>
+#include <stdio.h>
+
+int main(int argc, char** argv)
+{
+    if (argc == 1) {
+        fprintf(stderr, "usage: %s <binary>\n", argv[0]);
+        return 1;
+    }
+
+    MappedFile file(argv[1]);
+
+    X86::SimpleInstructionStream stream((const u8*)file.data(), file.size());
+    X86::Disassembler disassembler(stream);
+
+    for (;;) {
+        auto offset = stream.offset();
+        auto insn = disassembler.next();
+        if (!insn.has_value())
+            break;
+        out() << String::format("%08x", offset) << "  " << insn.value().to_string(offset, true);
+    }
+
+    return 0;
+}