toolchain-external: anchor sysroot regex with /

Anchor the regex in toolchain_find_sysroot macro with a / to avoid
unexpected substitution for Realtek mips toolchain, for which the libc.a
path ends with 'mips-linux-uclibc/lib/libc.a'.

Signed-off-by: 陈小 刚 <shawn_chen@realsil.com.cn>
Signed-off-by: Arnout Vandecappelle (Essensium/Mind) <arnout@mind.be>