testsuite: Prune compilation messages for modules tests

All testsuite compiler-calls pass default_target_compile in the
dejagnu installation (typically /usr/share/dejagnu/target.exp) which
also calls the dejagnu-installed prune_warnings.

Normally, tests using the dg framework (most or all tests these days)
compile and link by calling various wrappers that end up calling
dg-test in the dejagnu installation, typically installed as
/usr/share/dejagnu/dg.exp.  That, besides the compiler call, also
calls ${tool}-dg-prune (g++-dg-prune) on the messages, which in turn
ends up calling prune_gcc_output in gcc/testsuite/lib/prune.exp.  That
gcc-specific "pruning" function handles more cases than the dejagnu
prune_warnings, and also has updated patterns.

But, module_do_it in modules.exp calls the lower-level
${tool}_target_compile "directly", i.e. g++_target_compile defined in
gcc/testsuite/lib/g++.exp.  That does not call ${tool}-dg-prune,
meaning those test-cases miss the gcc-specific pruning.

Noticed while testing a dejagnu update that handled the miniscule "in"
in the warning (line-breaks added below besides the original one after
"(void*)':")

"/path/to/cris-elf/bin/ld:
/gccobj/cris-elf/./libstdc++-v3/src/.libs/libstdc++.a(random.o): in
function `std::(anonymous namespace)::__libc_getentropy(void*)':
/gccsrc/libstdc++-v3/src/c++11/random.cc:183: warning: _getentropy is
not implemented and will always fail"

The line saying "in function" rather than "In function" (from the
binutils linker since 2018) is pruned by prune_gcc_output. The
prune_warnings in dejagnu-1.6.3 and earlier handles the second line
separately.  It's an unfortunate wart that neither consumes the
delimiting line-break, leaving to the callers to prune residual empty
lines.  See prune_warnings in dejagnu (default_target_compile and
dg-test) for those other line-break fixups, as alluded in the comment.

	* g++.dg/modules/modules.exp (module_do_it): Prune compilation
	messages.