linux-low: Remove usage of "register" keyword

linux-low: Remove usage of "register" keyword

AFAIK, the register keyword is not relevant today, and clang complains
about it:

/home/emaisin/src/binutils-gdb/gdb/gdbserver/linux-low.c:5873:3: error: 'register' storage class specifier is deprecated and incompatible with C++1z
      [-Werror,-Wdeprecated-register]
  register PTRACE_XFER_TYPE *buffer;
  ^~~~~~~~~

I think we can safely remove it.

gdb/gdbserver/ChangeLog:

	* linux-low.c (linux_read_memory, linux_write_memory): Remove
	usage of "register" keyword.