testsuite: Add further zero size elt passing tests [PR102024]

testsuite: Add further zero size elt passing tests [PR102024]

As discussed in PR102024, zero width bitfields might not be the only ones
causing ABI issues at least on mips, zero size arrays or (in C only) zero
sized (empty) structures can be problematic too.

The following patch adds some coverage for it too.

Tested on x86_64-linux with
make check-gcc check-g++ RUNTESTFLAGS='ALT_CC_UNDER_TEST=gcc ALT_CXX_UNDER_TEST=g++ --target_board=unix\{-m32,-m64\} compat.exp=pr102024*'
make check-gcc check-g++ RUNTESTFLAGS='ALT_CC_UNDER_TEST=clang ALT_CXX_UNDER_TEST=clang++ --target_board=unix\{-m32,-m64\} compat.exp=pr102024*'
with gcc/g++ 10.3 and clang 11.  Everything but (expectedly)
FAIL: gcc.dg/compat/pr102024 c_compat_x_tst.o-c_compat_y_alt.o execute
FAIL: gcc.dg/compat/pr102024 c_compat_x_alt.o-c_compat_y_tst.o execute
for -m64 ALT_CC_UNDER_TEST=gcc passes.

2022-04-01  Jakub Jelinek  <jakub@redhat.com>

	PR target/102024
	* gcc.dg/compat/pr102024_test.h: Add further tests with zero sized
	structures and arrays.
	* g++.dg/compat/pr102024_test.h: Add further tests with zero sized
	arrays.