Makefile.boot: Make -Wno-typedef-redefinition Clang-specific

GCC doesn't have this warning and so also doesn't have the flag to
disable it, resulting in it spewing a bunch of warnings about the
command line option being unrecognised.

MFC after: 1 week

(cherry picked from commit 05066fad68f03f0f7cf602924dd35ee327bf2b2d)