docs: cxx requires both clang and lld

[zhouhaibing089]

While trying the guide on my archlinux desktop, I saw the following errors if I only have clang installed (via pacman -S clang)

$ buck2 build //...
Action failed: root//:main (cxx_link_executable)
Local command returned non-zero exit code 1
Reproduce locally: `clang++ "-fuse-ld=lld" -o buck-out/v2/gen/root/524f8da68ea2a374/__main__/main @buck-out/v2/gen/root/524f8da68ea2a374/__main__/main.linker.argsfile`
stdout:
stderr:
clang-15: error: invalid linker name in argument '-fuse-ld=lld'
Action failed: root//:print (cxx_link lib_print.so)
Local command returned non-zero exit code 1
Reproduce locally: `clang++ "-fuse-ld=lld" -o buck-out/v2/gen/root/524f8da68ea2a374/__print__/lib_print.so @buck-out/v2/gen/root/524f8da68ea2a374/__print__/lib_print.so.linker.argsfile`
stdout:
stderr:
clang-15: error: invalid linker name in argument '-fuse-ld=lld'
Build ID: c967e7ad-5e3c-49ac-8b92-bb5827cf9100
Jobs completed: 55. Time elapsed: 0.2s. Cache hits: 0%. Commands: 8 (cached: 0, remote: 0, local:
BUILD FAILED
Failed to build 'root//:main (prelude//platforms:default#524f8da68ea2a374)'
Failed to build 'root//:print (prelude//platforms:default#524f8da68ea2a374)'

It turns out that I need lld to be available as well (via pacman -S lld).

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[ndmitchell]

Thanks!