build-sys: fixup the build issue in 64bit system

In Debian/Ubuntu 64bit system, build error message:

......
......
/../libexec/gcc/arm-brcm-linux-uclibcgnueabi/4.5.3/cc1: error while
loading shared libraries: libmpc.so.2: cannot open shared object file:
No such file or directory make[3]: ***
......
......

We export a environment LD_LIBRARY_PATH to make the toolchain use the
own shared library libmpc2.so

Signed-off-by: Jack Tan <jiankemeng@gmail.com>