libhtml5.a has absolute header path

[eagleoflqj]

Step to reproduce:

• Use emscripten 4.0.0
• embuilder build libhtml5
• ar x libhtml5.a
• cat emscripten_wget.o
• See something like below, where /mnt/storage/github/emsdk is the place I clone emsdk.

.debug_strclang version 20.0.0git (https:/github.com/llvm/llvm-project 322eb1a92e6d4266184060346616fa0dbe39e731)/emsdk/emscripten/system/lib/html5/emscripten_wget.c/emsdk/emscriptenemscripten_wget_datacharintopenwritelongssize_tunsigned longsize_tclosefreestrdupmkdirunsigned intmode_t__errno_locationemscripten_wgetmkdirsbuffernumerrorfileurlfdcopycresult����
system/lib/html5cache/sysroot/include/emscriptencache/sysroot/includecache/sysroot/include/bits/mnt/storage/github/emsdk/upstream/lib/clang/20/includecache/sysroot/include/sysemscripten_wget.cwget.hfcntl.hunistd.halltypes.h__stddef_size_t.hstdlib.hstring.hstat.herrno.h

[sbc100]

At yes I think the problem is the clang include directory. For me that is /usr/local/google/home/sbc/dev/wasm/llvm-build/lib/clang/20/include. I guess we need an additional file-prefix-map in tools/system_libs.py?