gcc-runtime: Use static dummy libstdc++

some standalone targets e.g. riscv64-elf disable shared linking for
baremetal ELF ABI in ld, therefore lets make it a static library

Signed-off-by: Khem Raj <raj.khem@gmail.com>
Signed-off-by: Richard Purdie <richard.purdie@linuxfoundation.org>