docs: the toolchain page states that a stated linker is clang's, and its gcc example no longer states one

The cross example named `tools = { ld = ... }` on a gcc tree, which the engine now
refuses, and nothing said which trees read that role.

Author: speak-agent

docs/20-toolchains.md

@@ -386,7 +386,7 @@ prefix   = "aarch64-none-linux-gnu-"
 sysroot  = "/opt/acme-sysroot"
 family   = "gcc"                                 # checked against the drivers
 launcher = "ccache"                              # prefixes every compile
-tools    = { ld = "/opt/lld-dev/bin/ld.lld" }    # cc, cxx, ld, ar, ranlib, nm, objcopy, strip, as
+tools    = { ar = "/opt/acme-gcc/bin/gcc-ar" }   # cc, cxx, ar, ranlib, nm, objcopy, strip, as
 ```
 
 Or for one build, without editing the manifest:
@@ -402,6 +402,12 @@ whether `import std` is available are read from the driver itself. The tools
 beside them are found as `<prefix><tool>`, then `llvm-<tool>`, then `<tool>`;
 `tools` names any the tree does not have.
 
+`tools = { ld = … }` is read by a clang tree, where the linker reaches the link
+as `--ld-path`. A gcc tree that states `ld` is refused: gcc chooses its linker by
+the name `ld` inside a directory it is given with `-B`, so a program under any
+other name could not be selected, and a stated tool that took no part in the
+build is what this mechanism exists to prevent.
+
 **What mcpp does with it.** The same as with a payload it installed: its own
 link line, its own hermetic check, its own `import std` decision. It writes
 nothing into the tree — the generated `clang++.cfg` of a managed payload is

docs/zh/20-toolchains.md

@@ -364,7 +364,7 @@ prefix   = "aarch64-none-linux-gnu-"
 sysroot  = "/opt/acme-sysroot"
 family   = "gcc"                                 # 与驱动核对
 launcher = "ccache"                              # 前置于每次编译
-tools    = { ld = "/opt/lld-dev/bin/ld.lld" }    # cc、cxx、ld、ar、ranlib、nm、objcopy、strip、as
+tools    = { ar = "/opt/acme-gcc/bin/gcc-ar" }   # cc、cxx、ar、ranlib、nm、objcopy、strip、as
 ```
 
 或者只对一次构建生效，不改清单：
@@ -378,6 +378,10 @@ MCPP_TOOLCHAIN=path:/opt/llvm-trunk mcpp build
 得到。旁边的工具按 `<prefix><tool>`、`llvm-<tool>`、`<tool>` 查找；`tools` 点名树里没有的
 那些。
 
+`tools = { ld = … }` 只对 clang 的树成立——被陈述的链接器以 `--ld-path` 进入链接。gcc 的树
+陈述 `ld` 会被拒绝：gcc 按 `-B` 给出的目录里的名字 `ld` 选链接器，叫别的名字的程序无法那样
+被选中，而一个进了声明却不参与构建的工具，正是这套机制要防的事。
+
 **mcpp 如何驱动它。** 与它自己安装的载荷相同：自己的链接行、自己的 hermetic 检查、自己
 对 `import std` 的判定。它不向该目录树写入任何东西 —— 托管载荷里生成的 `clang++.cfg` 是
 mcpp 自己的文件，而一棵不属于 mcpp 的树不会得到一份。