[gdb/testsuite] Use regexp to match $_gdb_{major,minor}

[gdb/testsuite] Use regexp to match $_gdb_{major,minor}

Every time we update the gdb version number, test-case gdb.base/default.exp
needs updating because it matches the values of convenience variables
$_gdb_{major,minor} using hardcoded expected values:
...
	{$_gdb_major = 17} \
	{$_gdb_minor = 1} \
...

I'm assuming the values were hardcoded because gdb_test_list_exact was used.

Since the previous patch, that's not longer the case, so use regexps instead,
getting rid of this annoyance [1].

Tested on x86_64-linux.

[1] https://sourceware.org/pipermail/gdb-patches/2019-October/160935.html