rtdl: Return null when non-elf file is dlopened #3097

Job	Run time
Build mlibc (x86_64, mlibc)	16m 34s
Build mlibc (x86_64, mlibc-static)	2m 59s
Build mlibc (x86_64, mlibc-shared)	2m 54s
Build mlibc (x86_64, mlibc-ansi-only)	3m 7s
Build mlibc (x86_64, mlibc-headers-only)	1m 29s
Build mlibc (riscv64, mlibc)	3m 33s
Build mlibc (riscv64, mlibc-static)	7m 44s
Build mlibc (riscv64, mlibc-shared)	3m 32s
Build mlibc (riscv64, mlibc-ansi-only)	3m 33s
Build mlibc (riscv64, mlibc-headers-only)	1m 30s
Build mlibc (aarch64, mlibc)	5m 22s
Build mlibc (aarch64, mlibc-static)	3m 30s
Build mlibc (aarch64, mlibc-shared)	4m 21s
Build mlibc (aarch64, mlibc-ansi-only)	3m 48s
Build mlibc (aarch64, mlibc-headers-only)	1m 46s
Compile sysdeps (dripos)	1m 44s
Compile sysdeps (lemon)	2m 3s
Compile sysdeps (aero)	1m 50s
Compile sysdeps (ironclad)	1m 56s
Compile sysdeps (lyre)	1m 32s
Compile sysdeps (keyronex)	1m 52s
	1h 16m 39s

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