[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

gdb/testsuite/gdb.dwarf2/typeddwarf-amd64.S

@@ -116,9 +116,9 @@ f4:
 	.cfi_endproc
 .LFE3:
 	.size	f4, .-f4
-	.globl	_start
-	.type	_start, @function
-_start:
+	.globl	main
+	.type	main, @function
+main:
 .LFB4:
 	.loc 1 87 0
 	.cfi_startproc
@@ -170,7 +170,7 @@ _start:
 	ret
 	.cfi_endproc
 .LFE4:
-	.size	_start, .-_start
+	.size	main, .-main
 	.comm	vv,4,4
 	.section	.rodata.cst8,"aM",@progbits,8
 	.align 8

gdb/testsuite/gdb.dwarf2/typeddwarf.S

@@ -212,9 +212,9 @@ f4:
 	.size	f4, .-f4
 	.section	.text.startup,"ax",@progbits
 	.p2align 4,,15
-	.globl	_start
-	.type	_start, @function
-_start:
+	.globl	main
+	.type	main, @function
+main:
 .LFB4:
 	# typeddwarf.c:87
 .LM37:
@@ -314,7 +314,7 @@ _start:
 # SUCC: EXIT [100.0%] 
 	ret
 .LFE4:
-	.size	_start, .-_start
+	.size	main, .-main
 	.comm	vv,4,4
 	.section	.rodata.cst4,"aM",@progbits,4
 	.align 4

gdb/testsuite/gdb.dwarf2/typeddwarf.exp

@@ -31,7 +31,6 @@ if { [is_x86_like_target] } {
 
 set opts {}
 lappend opts nodebug
-lappend opts additional_flags=-nostdlib
 lappend opts nopie
 
 if { [prepare_for_testing "failed to prepare" "${test}" ${sfile} $opts] } {