Update glibc and newlib paths

Both the glibc and newlib submodules are already pointing to the
upstream repositories where development happens.
Since they are therefore not referring to RISC-V specific forks,
this patch unifies the directory naming by only prepending the
"riscv-" prefix to submodules that point to repositories in the
riscv-collab Github organization.

Signed-off-by: Florian Hofhammer <florian.hofhammer@fhofhammer.de>