This commit adds a __wrap_memcpy function and a linker flag to use that
in place of memcpy for our Ruby gem C extension. This allows us to
always use the 2.2.5 version of memcpy, making it possible to use the
gem on distributions with pre-2.14 versions of glibc.

Before this change:
$ objdump -T protobuf_c.so | grep memcpy
0000000000000000      DF *UND*  0000000000000000  GLIBC_2.3.4 __memcpy_chk
0000000000000000      DF *UND*  0000000000000000  GLIBC_2.14  memcpy

After:
$ objdump -T protobuf_c.so | grep memcpy
0000000000000000      DF *UND*  0000000000000000  GLIBC_2.2.5 memcpy
0000000000000000      DF *UND*  0000000000000000  GLIBC_2.3.4 __memcpy_chk
0000000000042450 g    DF .text  0000000000000005  Base __wrap_memcpy

This is based on gRPC's solution to a similar problem:
https://github.com/grpc/grpc/blob/5098508d2d41a116113f7e333c516cd9ef34a943/src/core/lib/support/wrap_memcpy.c

This fixes issue #2783.