build-scripts/build_llvm.py: bump to llvm 18 (#4259)

* build-scripts/build_llvm.py: bump to llvm 18

cf. https://github.com/bytecodealliance/wasm-micro-runtime/issues/4210

why not 20?
because, as of writing this, 19 is the latest released version for
the xtensa fork of llvm: https://github.com/espressif/llvm-project

why not 19?
because of a bug in the xtensa fork of llvm:
https://github.com/espressif/llvm-project/issues/112

while we can use different versions for different targets,
it's nicer to use the same version everywhere when possible.

* spec-test-script/runtest.py: --size-level=0 for x86-64

with the recent version of LLVM, wamrc --size-level=1 often
generates R_X86_64_32S relocations which fail on load with
the infamous error:

"relocation truncated to fit R_X86_64_32S failed"

it seems that these relocations are often for jump tables.

this commit workarounds it with --size-level=0.

an alternative is to disable jump tables. (although it seems that
jump tables are not the only source of these relocations.)

cf. https://github.com/bytecodealliance/wasm-micro-runtime/issues/3035

it might be better to do this in wamrc itself. however, currently
target info is not available there in case of native compilation.
related: https://github.com/bytecodealliance/wasm-micro-runtime/issues/3356

* wamr-compiler: size_level=0 for sgx mode

cf. https://github.com/bytecodealliance/wasm-micro-runtime/issues/3035

wamr-compiler/main.c

@@ -712,7 +712,7 @@ main(int argc, char *argv[])
     }
 
     if (sgx_mode) {
-        option.size_level = 1;
+        option.size_level = 0;
         option.is_sgx_platform = true;
     }