[OpenMP] Fix libomptarget test mapping/ompx_hold/struct.c

For me, the test fails for nvptx64 offload.  The problem was
introduced by D146838, which landed as 747af241.  It tries to copy
a string constant's address from device to host and then print the
string.  This patch copies the contents of the string instead.

Reviewed By: jdoerfert

Differential Revision: https://reviews.llvm.org/D149623