Clarify use of CC and friends for file generation

Add more detail around how generation of configuration-independent files
chooses a C compiler. Mention that setting HOSTCC or CC is recommended
where there are multiple toolchains.

Mention that the fallback location is the cc executable, which may help
users troubleshooting when the file generation picks up the wrong
toolchain (as in #10360).

Signed-off-by: David Horstmann <[email protected]>

Author: davidhorstmann-arm

README.md

@@ -72,7 +72,13 @@ The following tools are required:
     Depending on your Python installation, you may need to invoke `python` instead of `python3`. To install the packages system-wide, omit the `--user` option.
 * A C compiler for the host platform, for some test data.
 
-If you are cross-compiling, you must set the `CC` environment variable to a C compiler for the host platform when generating the configuration-independent files.
+The scripts that generate the configuration-independent files will look for a host C compiler in the following places (in order of preference):
+
+1. The `HOSTCC` environment variable. This can be used if `CC` is pointing to a cross-compiler.
+2. The `CC` environment variable.
+3. An executable called `cc` in the current path.
+
+Note: If you have multiple toolchains installed, it is recommended to set `CC` or `HOSTCC` to the intended host compiler before generating the files.
 
 Any of the following methods are available to generate the configuration-independent files: