[gdb/tdep] Support catch syscall pipe2 for i386

[gdb/tdep] Support catch syscall pipe2 for i386

With test-case gdb.base/catch-syscall.exp and target board unix/-m32, we run
into:
...
(gdb) catch syscall pipe2^M
Unknown syscall name 'pipe2'.^M
(gdb) FAIL: gdb.base/catch-syscall.exp: determine pipe syscall: catch syscall pipe2
...

Fix this by:
- adding a pipe2 entry in gdb/syscalls/i386-linux.xml.in, and
- regenerating gdb/syscalls/i386-linux.xml using
  "xsltproc --output i386-linux.xml apply-defaults.xsl i386-linux.xml.in".

Tested on x86_64-linux with native and unix/-m32.

Bug: https://sourceware.org/bugzilla/show_bug.cgi?id=29056