gdbserver arm: Add casts

gdbserver arm: Add casts

Trivial casts for C++.

Fixes things like

In file included from /home/simark/src/binutils-gdb/gdb/gdbserver/../common/common-defs.h:39:0,
                 from /home/simark/src/binutils-gdb/gdb/gdbserver/server.h:22,
                 from /home/simark/src/binutils-gdb/gdb/gdbserver/linux-arm-low.c:19:
/home/simark/src/binutils-gdb/gdb/gdbserver/linux-arm-low.c: In function ‘int arm_get_hwcap(long unsigned int*)’:
/home/simark/src/binutils-gdb/gdb/gdbserver/../../include/libiberty.h:711:38: error: invalid conversion from ‘void*’ to ‘unsigned char*’ [-fpermissive]
 # define alloca(x) __builtin_alloca(x)
                                      ^
/home/simark/src/binutils-gdb/gdb/gdbserver/linux-arm-low.c:807:25: note: in expansion of macro ‘alloca’
   unsigned char *data = alloca (8);
                         ^

gdb/gdbserver/ChangeLog:

	* linux-aarch32-low.c (arm_fill_gregset): Add cast.
	(arm_store_gregset): Likewise.
	* linux-arm-low.c (arm_get_hwcap): Likewise.
	(arm_read_description): Likewise.