gdb/testsuite/README: point to default value of INTERNAL_GDBFLAGS

The INTERNAL_GDBFLAGS runtest variable was updated in 55c3ad88
([gdb/testsuite] Prevent pagination in GDB_INTERNALFLAGS, 2020-10-26) to
disable pagination, and in aae1c79a (PR python/12227..., 2010-12-07)
to point to the data directory, but its default value mentioned in the
testsuite's README was not kept up to date.

To avoid it getting out of sync even more, point the reader to the
definition of the variable in lib/gdb.exp, and move the explanation of
the different flags there. Also adjust the example in the README
so it follows the flags added in 55c3ad88.

Change-Id: I3533608a7d6ae5198af09c7dc7743bde24c19ed7