add musl-clang, a wrapper for system clang installs

musl-clang allows the user to compile musl-powered programs using their
already existent clang install, without the need of a special cross compiler.
it achieves this by wrapping around both the system clang install and the
linker and passing them special flags to re-target musl at runtime.
it does only affect invocations done through the special musl-clang wrapper
script, so that the user setup remains fully intact otherwise.

the clang wrapper consists of the compiler frontend wrapper script,
musl-clang, and the linker wrapper script, ld.musl-clang.
musl-clang makes sure clang invokes ld.musl-clang to link objects; neither
script needs to be in PATH for the wrapper to work.
diff --git a/Makefile b/Makefile
index a1c1928..5f74005 100644
--- a/Makefile
+++ b/Makefile
@@ -52,6 +52,7 @@
 ALL_TOOLS = tools/musl-gcc
 
 WRAPCC_GCC = gcc
+WRAPCC_CLANG = clang
 
 LDSO_PATHNAME = $(syslibdir)/ld-musl-$(ARCH)$(SUBARCH).so.1
 
@@ -160,6 +161,10 @@
 	printf '#!/bin/sh\nexec "$${REALGCC:-$(WRAPCC_GCC)}" "$$@" -specs "%s/musl-gcc.specs"\n' "$(libdir)" > $@
 	chmod +x $@
 
+tools/%-clang: tools/%-clang.in config.mak
+	sed -e 's!@CC@!$(WRAPCC_CLANG)!g' -e 's!@PREFIX@!$(prefix)!g' -e 's!@INCDIR@!$(includedir)!g' -e 's!@LIBDIR@!$(libdir)!g' -e 's!@LDSO@!$(LDSO_PATHNAME)!g' $< > $@
+	chmod +x $@
+
 $(DESTDIR)$(bindir)/%: tools/%
 	$(INSTALL) -D $< $@