BUILD: Makefile: add linux-musl to TARGET

Other users are using musl, namely on Docker. It builds fine with
linux-glibc-legacy but not linux-glibc, which needs to first disable
USE_BACKTRACE. Better add a valid entry for it instead of hacking
around another libc.
diff --git a/INSTALL b/INSTALL
index 78424ee..6187892 100644
--- a/INSTALL
+++ b/INSTALL
@@ -372,6 +372,7 @@
 
   - linux-glibc         for Linux kernel 2.6.28 and above
   - linux-glibc-legacy  for Linux kernel 2.6.28 and above without new features
+  - linux-musl          for Linux kernel 2.6.28 and above with musl libc
   - solaris             for Solaris 8 or 10 (others untested)
   - freebsd             for FreeBSD 5 to 12 (others untested)
   - netbsd              for NetBSD