CCTools: Add backport for as -> clang exe path setting

This adds a backport of my accepted patch [0] to allow `as` to invoke the
correct `clang` executable.

[0] tpoechtrager/cctools-port#169

Author: staticfloat

bootstrap/CCTools/build_tarballs.jl

@@ -144,9 +144,15 @@ build_tarballs(;
     sources = [
         GitSource("https://github.com/tpoechtrager/cctools-port",
                   "2a3e1c2a6ff54a30f898b70cfb9ba1692a55fad7"),
+        DirectorySource("./patches"; target="patches"),
     ],
     script = raw"""
     cd ${WORKSPACE}/srcdir/cctools-port/cctools
+
+    for patch in ${WORKSPACE}/srcdir/patches/*.patch; do
+        atomic_patch -p2 ${patch}
+    done
+
     install_license ./APPLE_LICENSE
     install_license ./COPYING
     

bootstrap/CCTools/patches/as-clang-filename-envvar.patch

@@ -0,0 +1,18 @@
+diff --git a/cctools/as/driver.c b/cctools/as/driver.c
+index 8b642043..2ff04752 100644
+--- a/cctools/as/driver.c
++++ b/cctools/as/driver.c
+@@ -307,10 +307,12 @@ char **envp)
+ 	    as = makestr(prefix, CLANG, NULL);
+ #endif
+ 	    /* cctools-port start */
++		char *clang_exe = getenv("CCTOOLS_CLANG_AS_EXECUTABLE");
++		clang_exe = clang_exe ? clang_exe : "clang";
+ #ifndef __APPLE__
+ 	    char *target_triple = getenv("CCTOOLS_CLANG_AS_TARGET_TRIPLE");
+ #endif /* ! __APPLE__ */
+-	    as = find_executable("clang");
++	    as = find_executable(clang_exe);
+ 	    /* cctools-port end */
+ 	    if(!as || access(as, F_OK) != 0){ /* cctools-port: added  !as || */
+ 		printf("%s: assembler (%s) not installed\n", progname,