Remove testsuite compile errors with GCC5.

Remove testsuite compile errors with GCC5.

GCC5 defaults to the GNU11 standard for C and warns by default for
implicit function declarations and implicit return types.
https://gcc.gnu.org/gcc-5/porting_to.html

Fixing these issues in the testsuite turns 9 untested and 17 unsupported
testcases into 417 new passes when compiling with GCC5.

gdb/testsuite/ChangeLog:

        * gdb.arch/i386-bp_permanent.c (standard): New declaration.
        * gdb.base/disp-step-fork.c: Include unistd.h.
        * gdb.base/siginfo-obj.c: Include stdio.h.
        * gdb.base/siginfo-thread.c: Likewise.
        * gdb.mi/non-stop.c: Include unistd.h.
        * gdb.mi/nsthrexec.c: Include stdio.h.
        * gdb.mi/pthreads.c: Include unistd.h.
        * gdb.modula2/unbounded1.c (main): Declare returns int.
        * gdb.reverse/consecutive-reverse.c: Likewise.
        * gdb.threads/create-fail.c: Include unistd.h.
        * gdb.threads/killed.c: Likewise.
        * gdb.threads/linux-dp.c: Likewise.
        * gdb.threads/non-ldr-exc-1.c: Include stdio.h and string.h.
        * gdb.threads/non-ldr-exc-2.c: Likewise.
        * gdb.threads/non-ldr-exc-3.c: Likewise.
        * gdb.threads/non-ldr-exc-4.c: Likewise.
        * gdb.threads/pthreads.c: Include unistd.h.
        (main): Declare returns int.
        * gdb.threads/tls-main.c (foo): New declaration.
        * gdb.threads/watchpoint-fork-mt.c: Define _GNU_SOURCE.