improve detection of how to execute binaries on the host

[andrewrk]

from zig-specific options to generally recognized zig build options that any project can take advantage of. See the updated usage text for more details.

Prerequisite to #10143 (cc @nuald)

Next I want to look at getExternalExecutor and consider moving it from CrossTarget to Target, or otherwise resolve those TODO comments. There is also this:

$ grep -RI canExecBinariesOf ../src/ ../lib/
../src/main.zig:        if (!builtin.target.canExecBinariesOf(target)) {
../lib/std/target.zig:    pub fn canExecBinariesOf(host_target: Target, binary_target: Target) bool {

I think this should probably be removed and zig test would unconditionally try to execute the test binary. You'd have to pass --test-no-exec to disable this. As long as the error message is nice and clear this should be OK. Reasoning for this is that we want to keep awareness of qemu, wine, etc. in the zig build system and out of the zig compiler itself.

Also the code that calls getExternalExecutor is duplicated in std/build.zig and src/test.zig and I want to resolve that.

End goal here is that in a build.zig you could declaratively use a RunStep, without putting any logic inside build.zig to disable it. The RunStep itself would potentially emit an error if the build graph required it to be run, and there were no executors available to run it.

[andrewrk]

demo of 4f40d8e:

[nix-shell:~/dev/zig/build]$ uname -a
Linux ark 5.10.81 #1-NixOS SMP Sun Nov 21 12:46:37 UTC 2021 x86_64 GNU/Linux

[nix-shell:~/dev/zig/build]$ ./zig run ../test/standalone/hello_world/hello.zig -target aarch64-linux
warning: the host system (x86_64-linux.5.10.81...5.10.81-gnu.2.33) does not appear to be capable of executing binaries from the target (aarch64-linux.3.16...5.5.5-musl)
error: the following command failed to execve with 'InvalidExe':
/home/andy/.cache/zig/o/b61b3c9b78433477f6b5c343ce95631f/hello

[nix-shell:~/dev/zig/build]$ ./zig run ../test/standalone/hello_world/hello.zig -target x86_64-linux-gnu -lc
warning: the host system does not appear to be capable of executing binaries from the target because the host dynamic linker is located at '/nix/store/z56jcx3j1gfyk4sv7g8iaan0ssbdkhz1-glibc-2.33-56/lib/ld-linux-x86-64.so.2', while the target dynamic linker path is '/lib64/ld-linux-x86-64.so.2'. Consider using --dynamic-linker
error: the following command failed to execve with 'FileNotFound':
/home/andy/.cache/zig/o/f0f8bdc473cbcf7f00bd72f0450102ad/hello

[nix-shell:~/dev/zig/build]$ ./zig test ../test/behavior.zig -I ../test -target aarch64-linux 
warning: the host system (x86_64-linux.5.10.81...5.10.81-gnu.2.33) does not appear to be capable of executing binaries from the target (aarch64-linux.3.16...5.5.5-musl). Consider using --test-no-exec or --test-cmd
error: the following command failed with 'InvalidExe':
../test/zig-cache/o/b70989a95e10772fc12652c7f94a5cf5/test /home/andy/dev/zig/build/zig

[nix-shell:~/dev/zig/build]$ ./zig test ../test/behavior.zig -I ../test -target x86_64-linux-gnu -lc
warning: the host system does not appear to be capable of executing binaries from the target because the host dynamic linker is located at '/nix/store/z56jcx3j1gfyk4sv7g8iaan0ssbdkhz1-glibc-2.33-56/lib/ld-linux-x86-64.so.2', while the target dynamic linker path is '/lib64/ld-linux-x86-64.so.2'. Consider using --dynamic-linker, --test-no-exec, or --test-cmd
error: the following command failed with 'FileNotFound':
../test/zig-cache/o/795162652b84a90baa06fbb62db5eb68/test /home/andy/dev/zig/build/zig

[andrewrk]

I think the next step is going to be moving getExternalExecutor from CrossTarget to std.zig.system.Executor.detect and changing the fn signature from:

/// Note that even a `CrossTarget` which returns `false` for `isNative` could still be natively executed.
/// For example `-target arm-native` running on an aarch64 host.
pub fn getExternalExecutor(self: CrossTarget) Executor {

to

const NativeTargetInfo = std.zig.system.NativeTargetInfo;
pub fn detect(host: NativeTargetInfo, target: NativeTargetInfo) Executor {

This will make it correctly attempt to execute, e.g. 32-bit arm binaries on 64-bit arm hosts. It will also make it aware of dynamic linker incompatibilities.