Collect all function names to an array

Use a build script for `libm-test` to enumerate all symbols provided by
`libm` and provide this list in a variable. This will allow us to make
sure no functions are missed anytime they must be manually listed.

Additionally, introduce some helper config options.

library/compiler-builtins/libm/crates/libm-test/src/lib.rs

@@ -1 +1,2 @@
-
+// List of all files present in libm's source
+include!(concat!(env!("OUT_DIR"), "/all_files.rs"));