bootstrap: enable zlib for LLVM when we compile it for profiling

when compiling target LLVM with `opt-dist local` on Windows/MinGW,
profraw files are being compressed with zlib, so compiling without it
will make `llvm-profdata` complain about lacking of zlib support. this
error is shown:

`profile uses zlib compression but the profile reader was built without
zlib support`

example from llvm test suite: https://github.com/llvm/llvm-project/blob/76b5fcbf975547251faaeed8b567ea09d139a607/llvm/test/tools/llvm-profdata/nocompress.test#L15

Author: ognevny

src/bootstrap/src/core/build_steps/llvm.rs

@@ -370,8 +370,8 @@ impl Step for Llvm {
             cfg.define("LLVM_PROFDATA_FILE", path);
         }
 
-        // Libraries for ELF section compression.
-        if !target.is_windows() {
+        // Libraries for ELF section compression and profraw files merging.
+        if !target.is_windows() || builder.config.llvm_profile_generate {
             cfg.define("LLVM_ENABLE_ZLIB", "ON");
         } else {
             cfg.define("LLVM_ENABLE_ZLIB", "OFF");