Fix build failure on Linux amd64 with gcc 7
Fix build failure on Linux amd64 with gcc 7

Fixes this build failure:

/usr/bin/ld: ffi_stub_linux64.o: relocation R_X86_64_32S against
  symbol `c_callback_double_x64' can not be used when making a
  shared object; recompile with -fPIC

The stub now uses R_X86_64_REX_GOTPCRELX.

fujita-y/ypsilon,fujita-y/ypsilon