gdb: fix 'remote show FOO-packet' aliases

The following behaviour was observed in GDB:

  (gdb) show remote X-packet
  Support for the `p' packet is auto-detected, currently unknown.

Note the message mentions the 'p' packet.  This is a regression since
this commit:

  commit 8579fd13
  Date:   Mon Nov 8 14:58:46 2021 +0000

      gdb/gdbsupport: make xstrprintf and xstrvprintf return a unique_ptr

Before this commit the behaviour was:

  (gdb) show remote X-packet
  Support for the `X' packet is auto-detected, currently unknown.

The problem was caused by a failed attempt to ensure that some
allocated strings were deleted when GDB exits.  The code in the above
commit attempted to make use of 'static' to solve this problem,
however, the solution was just wrong.

In this new commit I instead allocate a static vector into which all
the allocated strings are stored, this ensures the strings are
released when GDB exits (which makes output from tools like valgrind
cleaner), but each string within the vector can be unique, which fixes
the regression.