Make <sys/types.h> be included prior to including <sys/user.h>.

glibc versions older than 2.14 use __uintNN_t types within certain
structures defined in <sys/user.h> probably assuming these types are
defined prior to including the header. This results in the following
`configure` feature test compilation error that makes it think that
`struct user_regs_struct` doesn't have `fs_base`/`gs_base` fields,
althouh it does.

    configure:13617: checking for struct user_regs_struct.fs_base
    configure:13617: gcc -c -g -O2 -I/linux/include conftest.c >&5
    In file included from conftest.c:158:0:
    /usr/include/sys/user.h:32:3: error: unknown type name '__uint16_t'
       __uint16_t  cwd;
       ^
    /usr/include/sys/user.h:33:3: error: unknown type name '__uint16_t'
       __uint16_t  swd;
       ^
    /usr/include/sys/user.h:34:3: error: unknown type name '__uint16_t'
       __uint16_t  ftw;
       ^
    /usr/include/sys/user.h:35:3: error: unknown type name '__uint16_t'
       __uint16_t  fop;
       ^
    /usr/include/sys/user.h:36:3: error: unknown type name '__uint64_t'
       __uint64_t  rip;
       ^
    /usr/include/sys/user.h:37:3: error: unknown type name '__uint64_t'
       __uint64_t  rdp;
       ^
    /usr/include/sys/user.h:38:3: error: unknown type name '__uint32_t'
       __uint32_t  mxcsr;
       ^
    /usr/include/sys/user.h:39:3: error: unknown type name '__uint32_t'
       __uint32_t  mxcr_mask;
       ^
    /usr/include/sys/user.h:40:3: error: unknown type name '__uint32_t'
       __uint32_t  st_space[32];   /* 8*16 bytes for each FP-reg = 128 bytes */
       ^
    /usr/include/sys/user.h:41:3: error: unknown type name '__uint32_t'
       __uint32_t  xmm_space[64];  /* 16*16 bytes for each XMM-reg = 256 bytes */
       ^
    /usr/include/sys/user.h:42:3: error: unknown type name '__uint32_t'
       __uint32_t  padding[24];
       ^
    configure:13617: $? = 1
    configure: failed program was:
    | /* confdefs.h */
    ...
    | /* end confdefs.h.  */
    | #include <sys/user.h>
    |
    | int
    | main ()
    | {
    | static struct user_regs_struct ac_aggr;
    | if (ac_aggr.fs_base)
    | return 0;
    |   ;
    |   return 0;
    | }

Recent glibc versions don't use typedef'ed int types in <sys/user.h>,
thus allowing it to be included as is
(glibc commit d79a9c949c84e7f0ba33e87447c47af833e9f11a).
However there're still some distros alive that use older glibc,
for instance, RHEL/CentOS 6 package glibc 2.12.

Also affects PR gdb/21559:

    ../../gdb/regcache.c:1087: internal-error: void regcache_raw_supply(regcache, int, const void): Assertion `regnum >= 0 && regnum < regcache->descr->nr_raw_registers' failed.

As noted by Andrew Paprocki, who submitted the PR
(https://sourceware.org/bugzilla/show_bug.cgi?id=21559#c3):

    > It should be noted that modifying `configure` to force on
    > `HAVE_STRUCT_USER_REGS_STRUCT_FS_BASE` and
    > `HAVE_STRUCT_USER_REGS_STRUCT_GS_BASE` fixes this issue. For some
    > reason the `configure` tests for `fs_base` and `gs_base` fail
    > even though `sys/user.h` on RHEL5 has the fields defined in
    > `user_regs_struct`.

Note that this patch does NOT fix the root cause of PR gdb/21559,
although now that `configure` properly detects the presence of the
fields and sets HAVE_XXX accordingly, the execution takes another
path, which doesn't lead to the assertion failure in question.

gdb/ChangeLog:
2018-01-17  Eldar Abusalimov  <eldar.abusalimov@jetbrains.com>

	PR gdb/21559
	* configure.ac: Include <sys/types.h> prior to <sys/user.h> when
	checking for fs_base/gs_base fields in struct user_regs_struct.
	* configure: Regenerate.

gdb/gdbserver/ChangeLog:
2018-01-17  Eldar Abusalimov  <eldar.abusalimov@jetbrains.com>

	PR gdb/21559
	* configure.ac: Include <sys/types.h> prior to <sys/user.h> when
	checking for fs_base/gs_base fields in struct user_regs_struct.
	* configure: Regenerate.