fix(cmake): avoid qjs/qjs_exe PDB filename collision on Windows parallel builds

[mvanhorn]

Summary

On Windows, when qjs (the executable) and qjs_exe (the helper target) are built in parallel, both targets write to qjs.pdb and collide. The fix sets a unique PDB_NAME per target.

Why this matters

Parallel-build collisions show up as flaky CL on Windows builders. Reporter pinpointed the colliding PDB filename in build logs.

Changes

• CMakeLists.txt - set PDB_NAME qjs for the qjs target and PDB_NAME qjs_exe for the qjs_exe target.

Fixes #1241

[mvanhorn]

Thanks for the quick merge, @saghul! Glad the PDB filename change cleared the Windows parallel-build collision.