Fix iov_len calculation in aarch64_linux_set_debug_regs

Fix iov_len calculation in aarch64_linux_set_debug_regs

There is this build failure when building in C++:

/home/simark/src/binutils-gdb/gdb/nat/aarch64-linux-hw-point.c: In function ‘void aarch64_linux_set_debug_regs(const aarch64_debug_reg_state*, int, int)’:
/home/simark/src/binutils-gdb/gdb/nat/aarch64-linux-hw-point.c:564:64: error: ‘count’ cannot appear in a constant-expression
   iov.iov_len = (offsetof (struct user_hwdebug_state, dbg_regs[count - 1])
                                                                ^
We can simplify the computation and make g++ happy at the same time by
formulating as:

  size of fixed part + size of variable part

thus...

  size of fixed part + count * size of one variable part element

thus...

  offsetof (struct user_hwdebug_state, dbg_regs) + count * sizeof (regs.dbg_reg[0]);

gdb/ChangeLog:

	* nat/aarch64-linux-hw-point.c (aarch64_linux_set_debug_regs): Change
	form of iov_len computation.