dub build: Fix cross-compilation

The build of the lexer dub subpackage requires running and building
a little config.d tool as custom pregenerate command.

Forwarding the target architecture via `--arch=$DUB_ARCH` to the
nested dub build of config.d was added in #9275, to fix cross-
compilation, but in reality broke it.

Not specifying the architecture explicitly for the little helper
build lets the compiler pick the default one, the host's native
platform in practice, which is guaranteed to be runnable on that
compiling **host**, independent from the **target** platform for
the main dub build. Suppose one cross-compiles the dmd:lexer
subpackage from x86_64 to AArch64 - an AArch64 config.d executable
will hardly run on the x86_64 host, and won't be able to generate
the `VERSION` and `SYSCONFDIR.imp` string-import files as pre-
requisite of the build.

Side note: using little separately-built .d tools/scripts as build
helpers for autogenerating little VERSION files etc. is IMO bad
practice - when cross-compiling, you require a D compiler that can
a) cross-compile, and b) build successfully for the native platform
too. Not sure this approach will e.g. ever work with GDC, where you
have different toolchains for each host->target combination.

dub.sdl

@@ -3,6 +3,7 @@ description "The DMD compiler"
 authors "Walter Bright"
 copyright "Copyright © 1999-2018, The D Language Foundation"
 license "BSL-1.0"
+toolchainRequirements dub=">=1.29.0" # can use $DUB_EXE etc. in custom commands
 
 targetType "none"
 dependency ":frontend" version="*"
@@ -50,17 +51,9 @@ subPackage {
     "CallbackAPI" \
     "DMDLIB"
 
-  preGenerateCommands `
-    "$${DUB_EXE}" \
-    --arch=$${DUB_ARCH} \
-    --compiler=$${DC} \
-    --single "$${DUB_PACKAGE_DIR}config.d" \
-    -- "$${DUB_PACKAGE_DIR}generated/dub" \
-    "$${DUB_PACKAGE_DIR}VERSION" \
-    /etc
-   ` platform="posix"
-
-  preGenerateCommands `"%DUB_EXE%" --arch=%DUB_ARCH% --compiler="%DC%" --single "%DUB_PACKAGE_DIR%config.d" -- "%DUB_PACKAGE_DIR%generated/dub" "%DUB_PACKAGE_DIR%VERSION"` platform="windows"
+  # generate files `generated/dub/{VERSION,SYSCONFDIR.imp}` for string-imports
+  # by building & running the config.d tool
+  preGenerateCommands `"$DUB_EXE" "--compiler=$DC" --single "${DUB_PACKAGE_DIR}config.d" -- "${DUB_PACKAGE_DIR}generated/dub" "${DUB_PACKAGE_DIR}VERSION" /etc`
 
   stringImportPaths \
     "compiler/src/dmd/res" \