fix gdb.base/jit-elf.exp when testing with clang

When using clang as the compiler for the target, gdb.base/jit-elf.exp
was failing because the filename displayed when GDB attached to the
inferior was only showing up as with a relative path, like so:

       (gdb) attach 3674146
       Attaching to program: /home/blarsen/Documents/gdb-build/gdb/testsuite/outputs/gdb.base/jit-elf/jit-elf-main, process 3674146
       Reading symbols from /lib64/libm.so.6...
       Reading symbols from .gnu_debugdata for /lib64/libm.so.6...
       (No debugging symbols found in .gnu_debugdata for /lib64/libm.so.6)
       Reading symbols from /lib64/libc.so.6...
       (No debugging symbols found in /lib64/libc.so.6)
       Reading symbols from /lib64/ld-linux-x86-64.so.2...
       [Thread debugging using libthread_db enabled]
       Using host libthread_db library "/lib64/libthread_db.so.1".
       0x00000000004013ff in main (argc=3, argv=0x7fffffffd820) at ../../../common/git-repos/binutils-gdb/gdb/testsuite/gdb.base/jit-elf-main.c:118
       118|  WAIT_FOR_GDB; i = 0;  /* gdb break here 1 */
       (gdb) FAIL: gdb.base/jit-elf.exp: attach: one_jit_test-2: break here 1: attach

While gcc's output is as follows:

       (gdb) attach 3592961
       Attaching to program: /home/blarsen/Documents/gdb-build/gdb/testsuite/outputs/gdb.base/jit-elf/jit-elf-main, process 3592961
       Reading symbols from /lib64/libm.so.6...
       Reading symbols from .gnu_debugdata for /lib64/libm.so.6...
       (No debugging symbols found in .gnu_debugdata for /lib64/libm.so.6)
       Reading symbols from /lib64/libc.so.6...
       (No debugging symbols found in /lib64/libc.so.6)
       Reading symbols from /lib64/ld-linux-x86-64.so.2...
       [Thread debugging using libthread_db enabled]
       Using host libthread_db library "/lib64/libthread_db.so.1".
       main (argc=3, argv=0x7fffffffd860) at /home/blarsen/Documents/gdb-build/gdb/testsuite/../../../common/git-repos/binutils-gdb/gdb/testsuite/gdb.base/jit-elf-main.c:118
       118|  WAIT_FOR_GDB; i = 0;  /* gdb break here 1 */
       (gdb) PASS: gdb.base/jit-elf.exp: attach: one_jit_test-2: break here 1: attach

This difference only happens when GDB's configure is ran using a
relative path, but seeing as testing the full path is not important for
this specific test, it feels worth fixing anyway.  To fix the false
positive, the regexp for checking where gdb has stopped was relaxed a
little to allow the relative path.

gdb/testsuite/gdb.base/jit-elf.exp

@@ -54,7 +54,7 @@ proc clean_reattach {} {
     clean_restart ${main_binfile}
 
     if { ![gdb_attach $testpid \
-	      -pattern "main.*at .*$::main_srcfile:.*"] } {
+	      -pattern "main.*at .*$::main_basename.c:.*"] } {
 	return 0
     }