Some toolchains will have string.h defining various macros which can lead
to compile errors for string function implementations. Not including
string.h fixes this.
An implementation of __memcpy_chk is provided for toolchains that enable
_FORTIFY_SOURCE.
Fixes issue #6046.
Signed-off-by: Alexey 'alexxy' Shvetsov <alexxyum@gmail.com>