Fix error when build elf32-only toolchain with --rel-rpaths

Script rel-rpaths.sh was trying to update uClibc-specific libc.so file
unconditionally. As a result elf32-only toolchain build was failing. This
patch adds a check that whether libc.so file actually exists before trying
to update it.

Signed-off-by: Anton Kolesov <akolesov@synopsys.com>