The ebuild of net-wireless/rtl8187 should check if the IEEE80211 Framework is compiled into the kernel, because net-wireless/rtl8187 comes with an own one that is not compatible to the one in the kernel and will crash the system with kernel-oops. After disabling the IEEE80211 Framework in the kernel, the rtl8187 driver works fine.
I added CONFIG_CHECK="!IEEE80211" to the ebuild, thanks.