syslinux: Pin to using GCC toolchain

It does not compile with clang due to include_next stdarg.h not working
as the system expects to match gcc behavior

(From OE-Core rev: 4ef959f37816f23e4ed57a71cb9a42fd818aa1fb)

Signed-off-by: Khem Raj <raj.khem@gmail.com>
Signed-off-by: Mathieu Dubois-Briand <mathieu.dubois-briand@bootlin.com>
Signed-off-by: Richard Purdie <richard.purdie@linuxfoundation.org>

meta/recipes-devtools/syslinux/syslinux_6.04-pre2.bb

@@ -130,3 +130,8 @@ FILES:${PN}-staticdev += "${datadir}/${BPN}/com32/lib*.a ${libdir}/${BPN}/com32/
 FILES:${PN}-misc = "${datadir}/${BPN}/* ${libdir}/${BPN}/* ${bindir}/*"
 
 BBCLASSEXTEND = "native nativesdk"
+
+# com32/lib/../include/stdarg.h:9:15: fatal error: 'stdarg.h' file not found
+#    9 | #include_next <stdarg.h>
+#      |               ^~~~~~~~~~
+TOOLCHAIN = "gcc"