eclass/toolchain.eclass: disable libstdcxx-time for bare metal

eclass/toolchain.eclass: disable libstdcxx-time for bare metal

Historically Gentoo unconditionally sets --enable-libstdcxx-time=yes
to avoid --enable-libstdcxx-time=rt on linux targets, bug #411681

Unfortunately this conflicts with arm-none-eabi and other
bare-metal targets that don't provide enough plumbing
to support for POSIX timers.

This change ogerrides our default to --disable-libstdcxx-time
on *-elf and *-eabi targets.

Tested as:
  $ crossdev --stage4 arm-none-eabi

Reported-by: scheer@wsoptics.de
Reported-by: Thomas Schneider
Bug: https://bugs.gentoo.org/411681
Bug: https://bugs.gentoo.org/589672
Signed-off-by: Sergei Trofimovich <slyfox@gentoo.org>