Sync proc_service definition with GLIBC

Sync proc_service definition with GLIBC

GLIBC BZ#20311 [1] proc_service.h install patch also remove 'const'
attributes from ps_get_thread_area and comment #15 discuss why to remove
the const attribute (basically since it a callback with the struct
ps_prochandle owned by the client it should be able to modify it if
it the case).

On default build this is not the issue and current g++ does not trigger
any issue with this mismatch declaration.  However, on some bootstrap
build configuration where gdbserver is build with gcc instead this
triggers:

error: conflicting types for 'ps_get_thread_area'

This patch fixes it by syncing the declaration with GLIBC.

[1] https://sourceware.org/bugzilla/show_bug.cgi?id=20311

gdb/ChangeLog:

2016-08-25  Adhemerval Zanella  <adhemerval.zanella@linaro.org>

	* aarch64-linux-nat.c (ps_get_thread_area): Remove const from
	struct ps_prochandle.
	* amd64-linux-nat.c (ps_get_thread_area): Likewise.
	* arm-linux-nat.c (ps_get_thread_area): Likewise.
	* gdb_proc_service.h (ps_get_thread_area): Likewise.
	* i386-linux-nat.c (ps_get_thread_area): Likewise.
	* m68klinux-nat.c (ps_get_thread_area): Likewise.
	* mips-linux-nat.c (ps_get_thread_area): Likewise.
	* nat/aarch64-linux.c (aarch64_ps_get_thread_area): Likewise.
	* nat/aarch64-linux.h (aarch64_ps_get_thread_area): Likewise.
	* xtensa-linux-nat.c (ps_get_thread_area): Likewise.

gdb/gdbserver/ChangeLog:

2016-08-25  Adhemerval Zanella  <adhemerval.zanella@linaro.org>

	PR server/20491
	* gdb_proc_service.h (ps_get_thread_area): Remove const from struct
	ps_prochandle.
	* linux-aarch64-low.c (ps_get_thread_area): Likewise.
	* linux-arm-low.c (ps_get_thread_area): Likewise.
	* linux-crisv32-low.c (ps_get_thread_area): Likewise.
	* linux-m68k-low.c (ps_get_thread_area): Likewise.
	* linux-mips-low.c (ps_get_thread_area): Likewise.
	* linux-nios2-low.c (ps_get_thread_area): Likewise.
	* linux-tic6x-low.c (ps_get_thread_area): Likewise.
	* linux-x86-low.c (ps_get_thread_area): Likewise.
	* linux-xtensa-low.c (ps_get_thread_area): Likewise.