Work around gcc10 FAIL: S-records with constructors

gcc10 on x86_64 and powerpc64le recognises that the loop in
Foo::operator= can be turned into a call to memmove, which then
results in an undefined symbol when linking.  Avoid that by making the
loop smaller.

	* testsuite/ld-srec/sr3.cc (FOO_MSG_LEN): Set to 4.