Use a non-empty test program to test ability to link.
On bare-metal targets, I/O support is typically provided by a BSP and
requires a linker script and/or hosting library to be specified on the
linker command line. Linking an empty program with the default linker
script may succeed, however, which confuses libstdc++ configuration
when programs that probe for the presence of various I/O features fail
with link errors.