Description:
The musl-clang wrapper breaks when a C file includes <immintrin.h>.
This is due to the wrapper disabling system and compiler include paths entirely with '-nolibinc'.

Additional info:
* musl 1.2.3-1

Steps to reproduce:
- Install 'musl' and 'clang' packages and try:
$ echo -e "#include <immintrin.h>\nint main(){return 0;}" | clang -x c -
^ works
$ echo -e "#include <immintrin.h>\nint main(){return 0;}" | musl-clang -x c -
^ breaks

Steps to fix:
Replace '-nolibinc' with '-nostdlibinc' (description in clang manpage) in wrapper so system include directories like '/usr/include' are still not searched, but compiler ones like '/usr/lib/clang/$clang_ver/include' that contain immintrin.h, etc. are.
Add just "make DESTDIR="$pkgdir" install":
sed -e "s/-nostdinc/-nostdlibinc/g" -i "${pkgdir}/usr/bin/musl-clang"

I am not sure if this could break something else, but in my opinion compiler include directories should be essential for the compiler to work properly.