track prefix toolchain tarballs for disaster recovery

make distclean destroyed prefix/ which was entirely gitignored.
Now prefix/*.tar.gz (clang/gcc/rust toolchain, ~408MB) is tracked.
Recovery after distclean: git checkout + make prefix (extract + build relibc).

prefix/.gitignore allows only *.tar.gz files; extracted directories,
relibc-install, and sysroot remain untracked (regenerable).

.gitignore

@@ -1,5 +1,4 @@
 /build/
-/prefix/
 **/my_*
 
 .idea/

prefix/.gitignore

@@ -0,0 +1,8 @@
+# Track: toolchain tarballs for instant recovery after make distclean
+# Ignore: extracted directories (regenerable), sysroot (changes per build), relibc-install
+
+# Ignore everything by default
+/*
+
+# Track tarballs
+!/*.tar.gz