perl: Fix various nativesdk build issues

perl: Fix various nativesdk build issues

The config.sh lists -fstack-protector but this isn't in LDFLAGS. This
can result in perl compilation failures due to the mismatch. Adding the
flag to LDFLAGS solves makes all the flags consistent and avoids build
failures from missing symbols.

It was also found that the path substitutions made by the sed statement
can conflict with each other and you can end up with $prefix$prefix type
expressions in config.sh-X which can break the build in unusual ways.
This patch anchors the expressions to ensure only true matches are
replaced.

(From OE-Core rev: 38234f2e276356b1d77a87ceabc486107e336d19)

Signed-off-by: Richard Purdie <richard.purdie@linuxfoundation.org>