[gcov] Improve tests and lower the minimum supported version to gcov 3.4

global-ctor.ll no longer checks what it intended to check
(@_GLOBAL__sub_I_global-ctor.ll needs a !dbg to work).
Rewrite it.

gcov 3.4 and gcov 4.2 use the same format, thus we can lower the version
requirement to 3.4