Use arrays rather than pointers for global string constants

My understanding is that it's mildly better to use a static const
array, as opposed to a "const char *", for a global string constant,
when possible.  This makes sense to me because the pointer requires a
load from an address, whereas the array is just the address.

So, I searched for these in gdb and gdbserver.  This patch fixes the
ones I found.

gdb/ChangeLog
2020-09-15  Tom Tromey  <tromey@adacore.com>

	* unittests/memory-map-selftests.c (valid_mem_map): Now array.
	* ui-style.c (ansi_regex_text): Now array.
	* rust-exp.y (number_regex_text): Now array.
	* linespec.c (linespec_quote_characters): Now array.
	* jit.c (jit_break_name, jit_descriptor_name, reader_init_fn_sym):
	Now arrays.

gdbserver/ChangeLog
2020-09-15  Tom Tromey  <tromey@adacore.com>

	* linux-x86-low.cc (xmltarget_i386_linux_no_xml)
	(xmltarget_amd64_linux_no_xml): Now arrays.