#include<stdio.h>

int main() {
    printf("hallo");
}

clang -c -emit-llvm main.c
opt -O3 main.bc -o optimized.bc
llc -march=x86-64 -filetype=obj optimized.bc -o prog.o
ld --verbose -static -lc prog.o -o prog --entry main

...
attempt to open //usr/local/lib/x86_64-linux-gnu/libc.a failed
attempt to open //lib/x86_64-linux-gnu/libc.a failed
attempt to open //usr/lib/x86_64-linux-gnu/libc.a succeeded
attempt to open prog.o succeeded
prog.o
prog.o: In function `main':
optimized.bc:(.text+0xc): undefined reference to `printf'

int main() {
    int a = 5;
}