[gdb/testsuite] Fix gdb.base/attach-deleted-exec.exp with NFS

[gdb/testsuite] Fix gdb.base/attach-deleted-exec.exp with NFS

With test-case gdb.base/attach-deleted-exec.exp I ran into:
...
(gdb) attach 121552^M
Attaching to process 121552^M
Reading symbols .../attach-deleted-exec/.nfs00000000044ff2ef00000086...^M
Reading symbols from /lib64/libm.so.6...^M
(No debugging symbols found in /lib64/libm.so.6)^M
Reading symbols from /lib64/libc.so.6...^M
(No debugging symbols found in /lib64/libc.so.6)^M
Reading symbols from /lib64/ld64.so.2...^M
(No debugging symbols found in /lib64/ld64.so.2)^M
0x00007fff947cc838 in clock_nanosleep@@GLIBC_2.17 () from /lib64/libc.so.6^M
(gdb) FAIL: $exp: attach to process with deleted executable
....

The .nfs file indicates:
- that the file has been removed on the NFS server, and
- that the file is still open on the NFS client.

Fix this by detecting this situation, and declaring the test for filename
/proc/PID/exe unsupported.

Tested on:
- x86_64-linux (setup without NFS)
- ppc64le-linux (setup with NFS)

PR testsuite/32130
Bug: https://sourceware.org/bugzilla/show_bug.cgi?id=32130