Fix siginfo C++ build error

Fix siginfo C++ build error

Change the signature of gdbserver's siginfo_fixup functions so that it's
in line with gdb's.  This gets rid of the following build error in C++:

  /home/emaisin/src/binutils-gdb/gdb/gdbserver/linux-x86-low.c: In function ‘int x86_siginfo_fixup(siginfo_t*, void*, int)’:
  /home/emaisin/src/binutils-gdb/gdb/gdbserver/linux-x86-low.c:694:21: error: invalid conversion from ‘void*’ to ‘gdb_byte* {aka unsigned char*}’ [-fpermissive]
               FIXUP_32);
                       ^
  In file included from /home/emaisin/src/binutils-gdb/gdb/gdbserver/linux-x86-low.c:31:0:
  /home/emaisin/src/binutils-gdb/gdb/gdbserver/../nat/amd64-linux-siginfo.h:52:5: error:   initializing argument 2 of ‘int amd64_linux_siginfo_fixup_common(siginfo_t*, gdb_byte*, int, amd64_siginfo_fixup_mode)’ [-fpermissive]
   int amd64_linux_siginfo_fixup_common (siginfo_t *native, gdb_byte *inf,
       ^
  /home/emaisin/src/binutils-gdb/gdb/gdbserver/linux-x86-low.c:698:20: error: invalid conversion from ‘void*’ to ‘gdb_byte* {aka unsigned char*}’ [-fpermissive]
             FIXUP_X32);
                      ^
  In file included from /home/emaisin/src/binutils-gdb/gdb/gdbserver/linux-x86-low.c:31:0:
  /home/emaisin/src/binutils-gdb/gdb/gdbserver/../nat/amd64-linux-siginfo.h:52:5: error:   initializing argument 2 of ‘int amd64_linux_siginfo_fixup_common(siginfo_t*, gdb_byte*, int, amd64_siginfo_fixup_mode)’ [-fpermissive]
   int amd64_linux_siginfo_fixup_common (siginfo_t *native, gdb_byte *inf,
       ^

gdb/gdbserver/ChangeLog:

	* linux-aarch64-low.c (aarch64_linux_siginfo_fixup): Change
	void * to gdb_byte *.
	* linux-low.c (siginfo_fixup): Likewise.
	(linux_xfer_siginfo): Likewise.
	* linux-low.h (struct linux_target_ops) <siginfo_fixup>:
	Likewise.
	* linux-x86-low.c (x86_siginfo_fixup): Likewise.