build-llvm: Fix using a relative PREFIX path

This fixes a regression from
6d31f358d8, which shows up as
"./strip-llvm.sh: 129: cd: can't cd to share" when running
build-all.sh, fixing
https://github.com/mstorsjo/llvm-mingw/issues/509, and the root
issue behind https://github.com/mstorsjo/llvm-mingw/pull/507.

build-llvm.sh

@@ -108,7 +108,7 @@ if [ -z "$CHECKOUT_ONLY" ]; then
         exit 1
     fi
 
-    if [ -z "$INSTRUMENTED" ]; then
+    if [ "$INSTRUMENTED" = "OFF" ]; then
         mkdir -p "$PREFIX"
         PREFIX="$(cd "$PREFIX" && pwd)"
     fi