rules.mak: Use -r instead of -Wl, -r to fix building when PIE is default

Building qemu fails in distributions where gcc enables PIE by default
(e.g. Debian unstable) with:

/usr/bin/ld: -r and -pie may not be used together

Use -r instead of -Wl,-r to avoid gcc passing -pie to the linker
when PIE is enabled and a relocatable object is passed.

Signed-off-by: Adrian Bunk <bunk@stusta.de>
Message-Id: <20161127162817.15144-1-bunk@stusta.de>
Cc: qemu-stable@nongnu.org
Signed-off-by: Paolo Bonzini <pbonzini@redhat.com>