[gdb/cli] Warn about forced return from signal trampoline

The Linaro CI reported a regression on arm-linux in test-case
gdb.base/sigstep.exp following commit 7b46460a619 ("[gdb/symtab] Apply
workaround for PR gas/31115 a bit more") [1]:
...
(gdb) return^M
Make __default_sa_restorer return now? (y or n) n^M
Not confirmed^M
(gdb) FAIL: $exp: return from handleri: \
  leave signal trampoline (got interactive prompt)
...

After installing package glibc-debuginfo and adding --with-separate-debug-dir
to the configure flags, I managed to reproduce the FAIL.

The regression seems to be a progression in the sense that the function name
for the signal trampoline is found.

After reading up on the signal trampoline [2] and the return command [3], my
understanding is that forced returning from the signal trampoline is
potentially unsafe, given that for instance the process signal mask won't be
restored.

Fix this by:
- rather than using the name, using "signal trampoline" in the query, and
- adding a warning about returning from a signal trampoline,
giving us:
...
(gdb) return^M
warning: Returning from signal trampoline does not fully restore pre-signal \
  state, such as process signal mask.^M
Make signal trampoline return now? (y or n) y^M
87            dummy = 0; dummy = 0; while (!done);^M
(gdb) PASS: $exp: return from handleri: leave signal trampoline (in main)
...

Tested on x86_64-linux.

Reviewed-by: Thiago Jung Bauermann <thiago.bauermann@linaro.org>

[1] https://linaro.atlassian.net/browse/GNU-1459
[2] https://man7.org/linux/man-pages/man2/sigreturn.2.html
[3] https://sourceware.org/gdb/current/onlinedocs/gdb.html/Returning.html

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