Resolve $PREFIX to an absolute path at the start of shell scripts

This fixes building/installing into a relative path.

build-compiler-rt.sh

@@ -19,6 +19,9 @@ if [ -z "$PREFIX" ]; then
     echo $0 [--build-sanitizers] dest
     exit 1
 fi
+
+mkdir -p "$PREFIX"
+PREFIX="$(cd "$PREFIX" && pwd)"
 export PATH=$PREFIX/bin:$PATH
 
 : ${CORES:=$(nproc 2>/dev/null)}