User-provided `-isystem` ordered after Zig-provided libc

[topolarity]

Zig Version

0.11.0-dev.106+e53a542e0

Steps to Reproduce and Observed Behavior

clang -xc -isystem test -E /dev/null -v shows -isystem at the top of the system search list:

#include <...> search starts here:
 test
 /usr/lib/llvm-14/lib/clang/14.0.6/include
 /usr/local/include
 /usr/include/x86_64-linux-gnu
 /usr/include

Running zig build-obj foo.c -isystem test --verbose-cc and then adding -v to the invocation shows that Zig's stdlib and host-detected libc directories appear first:

#include <...> search starts here:
 /home/topolarity/repos/zig/lib/include
 /usr/include
 /usr/include/x86_64-linux-gnu
 test

Expected Behavior

Behavior should match clang - automatic standard library includes should come after user-provided ones.

This ordering is required to be able to provide libc++ via -isystem (e.g. from the LLVM docs)

Related: https://github.com/ziglang/zig/issues/12888