gdb/testsuite: use test_compiler_info in gcc_major_version

gdb/testsuite: use test_compiler_info in gcc_major_version

The procedure gcc_major_version was earlier using the global variable
compiler_info to retrieve gcc's major version.  This is discouraged and
(as can be read in a comment in compiler.c) compiler_info should be
local to get_compiler_info and test_compiler_info.

The preferred way of getting the compiler string is via calling
test_compiler_info without arguments.  Gcc_major_version was changed to
do that.