[gdb/symtab] Support -readnow during reread

When running test-case gdb.base/cached-source-file.exp with target board
readnow, we run into:
...
FAIL: gdb.base/cached-source-file.exp: rerun program (the program exited)
...

The problem is that when rereading, the readnow is ignored.

Fix this by copying the readnow handling code from symbol_file_add_with_addrs
to reread_symbols.

Tested on x86_64-linux.

Bug: https://sourceware.org/bugzilla/show_bug.cgi?id=26800