[gdb/testsuite] Use -nostdlib in gdb.linespec/explicit.exp

[gdb/testsuite] Use -nostdlib in gdb.linespec/explicit.exp

On openSUSE Leap 15.6 ppc64le-linux, with gdb.linespec/explicit.exp I run
into:
...
(gdb) b -source thread_pointer.h FAIL: $exp: complete after -source: tab complete "b -source thr"
Quit^M
...

The test-case already contains a related workaround:
...
	# Get rid of symbols from shared libraries, otherwise
	# "b -source thr<tab>" could find some system library's
	# source.
	gdb_test_no_output "nosharedlibrary"
...
but that doesn't work in this case because the debug info is in the executable
itself:
...
 The File Name Table (offset 0xb5):
  Entry Dir     Time    Size    Name
  1     0       0       0       abi-note.c
  2     1       0       0       types.h
  3     2       0       0       stdint-intn.h
  4     2       0       0       stdint-uintn.h
  5     3       0       0       elf.h
  6     4       0       0       thread_pointer.h
...
due to debug info in some glibc object file.

Fix this by:
- using -nostdlib, ensuring only debug info from the three test-case sources
  is present in the executable, and
- adding a _start wrapping main.

Tested on x86_64-linux and ppc64le-linux.

Reviewed-By: Keith Seitz <keiths@redhat.com>
PR testsuite/31229
Bug: https://sourceware.org/bugzilla/show_bug.cgi?id=31229