[gdb/testsuite] Drop -nostdlib in gdb.dwarf2/typeddwarf.exp

[gdb/testsuite] Drop -nostdlib in gdb.dwarf2/typeddwarf.exp

As reported in PR testsuite/30633, when running test-case
gdb.dwarf2/typeddwarf.exp with target board native-gdbserver on Ubuntu
22.04.2, we run into:
...
(gdb) continue^M
Continuing.^M
^M
Program received signal SIGSEGV, Segmentation fault.^M
0x0000000000000001 in ?? ()^M
(gdb) FAIL: gdb.dwarf2/typeddwarf.exp: runto: run to main
...

We run into the FAIL as follows:
- due to using gdbserver, we attach at the point of the first instruction, in
  _start
- we then set a breakpoint at main
- the test-case is a .s file, that has main renamed to _start in the assembly,
  but not in the debuginfo
- setting a breakpoint at main sets the breakpoint at the same instruction
  we're currently stopped at
- continue doesn't hit the breakpoint, and we return out of _start, which
  causes a sigsegv

Note that this is for the amd64 case (using gdb.dwarf2/typeddwarf-amd64.S).
For the i386 case (using gdb.dwarf2/typeddwarf.S), setting a breakpoint in
main sets it one insn after function entry, and consequently the problem does
not occur.

The FAIL is a regression since commit 90cce6c0551 ("[gdb/testsuite] Add nopie
in a few test-cases").

Without nopie the executable is PIE, with nopie it's static instead.

In the PIE case, we attach at the point of _start in the dynamic linker, and
consequently we do not skip the breakpoint in main, and also don't run into
the FAIL.

Fix this by:
- removing the -nostdlib setting, and
- renaming _start to main in both .S files.

The change to use -nostdlib and rename main to _start was originally added
in commit 6edba76fe8b (submitted here:
https://sourceware.org/pipermail/gdb-patches/2011-May/082657.html ) , I assume
to fix the problem now fixed by using nopie.

Tested on x86_64-linux.

Reported-By: Simon Marchi <simon.marchi@efficios.com>
Tested-By: Simon Marchi <simon.marchi@efficios.com>
Bug: https://sourceware.org/bugzilla/show_bug.cgi?id=30633