luigi ~ # emerge -D libstdc
Calculating dependencies   
emerge: there are no ebuilds to satisfy "libstdc".

luigi ~ # emerge -D sys-libs/libstdc++-v3
Calculating dependencies   
!!! All ebuilds that could satisfy "sys-libs/libstdc++-v3" have been masked.
!!! One of the following masked packages is required to complete your request:
- sys-libs/libstdc++-v3-3.3.3-r1 (masked by: ~x86 keyword)
- sys-libs/libstdc++-v3-3.3.4 (masked by: ~x86 keyword)
- sys-libs/libstdc++-v3-3.3.6 (masked by: ~x86 keyword)

For more information, see MASKED PACKAGES section in the emerge man page or 
section 2.2 "Software Availability" in the Gentoo Handbook.