emulators/qemu: Fix build on 13

• Use ports dtc instead of packaged dtc [1]
• bin/elf2dmp apparently depends on CURL [2]
• Reduce build to only X86 architectures if X86_TARGETS is set [2]
• rx is not an X86 architecture [2]

PR: 245997 [1] 254385 [2]
Reported by: swills [1] pstef [2]