various: fix hostPlatform.canExecute buildPlatform

It doesn't matter whether the hostPlatform could execute stuff from the
buildPlatform - we need it the other way around: Certain things during a
build process can only happen when, as part of the build, the
**buildPlatform** can execute the code it just created for the
**hostPlatform**.

Unless we are talking about compilation at run-time, as in the case of
PostgreSQL. Extend the comment there to make it clear that this is on
purpose.

pkgs/by-name/li/libnvme/package.nix

@@ -14,7 +14,7 @@
   swig,
   systemd,
   # ImportError: cannot import name 'mlog' from 'mesonbuild'
-  withDocs ? stdenv.hostPlatform.canExecute stdenv.buildPlatform,
+  withDocs ? stdenv.buildPlatform.canExecute stdenv.hostPlatform,
 }:
 
 stdenv.mkDerivation (finalAttrs: {

pkgs/by-name/re/repgrep/package.nix

@@ -37,7 +37,7 @@ rustPlatform.buildRustPackage rec {
       installManPage rgr.1
       popd
     ''
-    + lib.optionalString (stdenv.hostPlatform.canExecute stdenv.buildPlatform) ''
+    + lib.optionalString (stdenv.buildPlatform.canExecute stdenv.hostPlatform) ''
       # As it can be seen here: https://github.com/acheronfail/repgrep/blob/0.15.0/.github/workflows/release.yml#L206, the completions are just the same as ripgrep
       installShellCompletion --cmd rgr \
         --bash <(${lib.getExe ripgrep} --generate complete-bash | sed 's/-c rg/-c rgr/') \

pkgs/by-name/vl/vlc/package.nix

@@ -253,7 +253,7 @@ stdenv.mkDerivation (finalAttrs: {
     # Upstream luac can't cross compile, so we have to install the lua sources
     # instead of bytecode:
     # https://www.lua.org/wshop13/Jericke.pdf#page=39
-    + lib.optionalString (!stdenv.hostPlatform.canExecute stdenv.buildPlatform) ''
+    + lib.optionalString (!stdenv.buildPlatform.canExecute stdenv.hostPlatform) ''
       substituteInPlace share/Makefile.am \
         --replace $'.luac \\\n' $'.lua \\\n'
     '';

pkgs/servers/sql/postgresql/generic.nix

@@ -407,6 +407,9 @@ let
         # resulting LLVM IR isn't platform-independent this doesn't give you much.
         # In fact, I tried to test the result in a VM-test, but as soon as JIT was used to optimize
         # a query, postgres would coredump with `Illegal instruction`.
+        #
+        # Note: This is "host canExecute build" on purpose, since this is about the LLVM that is called
+        # to do JIT at **runtime**.
         broken = jitSupport && !stdenv.hostPlatform.canExecute stdenv.buildPlatform;
       };
     });