Revert "llvm: Upgrade to LLVM 18.1.6"

This is to avoid the below verifier issue in cilium/cilium

```
    --- FAIL: TestVerifier/bpf_host (111.35s)
        --- FAIL: TestVerifier/bpf_host/1 (24.05s)
        --- FAIL: TestVerifier/bpf_host/2 (21.32s)
	...
```

Sample error log for TestVerifier/bpf_host/1

```
    verifier_test.go:244: Error: program tail_nodeport_nat_ingress_ipv4: load program: argument list too long: BPF program is too large. Processed 1000001 insn (1445 line(s) omitted)
        Verifier error tail: load program: argument list too long:
        	(1436 line(s) omitted)
        	; struct ipv4_ct_tuple icmp_tuple = {
        	1201: (63) *(u32 *)(r10 -12) = r1
        	; .flags = tuple->flags | TUPLE_F_RELATED,
        	1202: (71) r1 = *(u8 *)(r10 -123)
        	; .flags = tuple->flags | TUPLE_F_RELATED,
        	1203: (44) w1 |= 2
        	; struct ipv4_ct_tuple icmp_tuple = {
        	1204: (73) *(u8 *)(r10 -3) = r1
        	BPF program is too large. Processed 1000001 insn
        	processed 1000001 insns (limit 1000000) max_states_per_insn 17 total_states 55755 peak_states 1015 mark_read 59
```

Sample error log for TestVerifier/bpf_host/2
```
    verifier_test.go:244: Error: program tail_nodeport_nat_ingress_ipv4: load program: argument list too long: BPF program is too large. Processed 1000001 insn (1445 line(s) omitted)
        Verifier error tail: load program: argument list too long:
        	(1436 line(s) omitted)
        	; struct ipv4_ct_tuple icmp_tuple = {
        	1201: (63) *(u32 *)(r10 -12) = r1
        	; .flags = tuple->flags | TUPLE_F_RELATED,
        	1202: (71) r1 = *(u8 *)(r10 -123)
        	; .flags = tuple->flags | TUPLE_F_RELATED,
        	1203: (44) w1 |= 2
        	; struct ipv4_ct_tuple icmp_tuple = {
        	1204: (73) *(u8 *)(r10 -3) = r1
        	BPF program is too large. Processed 1000001 insn
        	processed 1000001 insns (limit 1000000) max_states_per_insn 17 total_states 55755 peak_states 1015 mark_read 59
```

This reverts commit 11cfa1a.

images/llvm/checkout-llvm.sh

@@ -8,7 +8,7 @@ set -o errexit
 set -o pipefail
 set -o nounset
 
-rev="llvmorg-18.1.6"
+rev="llvmorg-17.0.6"
 
 git config --global user.email "[email protected]"
 git config --global user.name  "Cilium Maintainers"

images/llvm/test/spec.yaml

@@ -10,7 +10,7 @@ commandTests:
   args: ["--version"]
   expectedOutput:
   - 'LLVM\ \(http://llvm\.org/\):'
-  - 'LLVM\ version\ 18\.1\.6'
+  - 'LLVM\ version\ 17\.0\.6'
   - 'Optimized\ build\.'
   - 'Registered\ Targets:'
   - '(bpf|bpfeb|bpfel)[\ ]+-\ BPF\ \((host|big|little)\ endian\)'
@@ -24,7 +24,7 @@ commandTests:
   command: "clang"
   args: ["--version"]
   expectedOutput:
-  - 'clang\ version\ 18\.1\.6\ \(https://github.com/llvm/llvm-project.git\ .*\)'
+  - 'clang\ version\ 17\.0\.6\ \(https://github.com/llvm/llvm-project.git\ .*\)'
   - 'InstalledDir:\ /usr/local/bin'
 
 - name: "clang can compile a simple BPF program"