ia64-tdep.c: Fix -Wwrite-strings errors

ia64-tdep.c: Fix -Wwrite-strings errors

Simply add const where necessary to get rid of errors like:

/home/simark/src/binutils-gdb/gdb/ia64-libunwind-tdep.c:114:1: error: ISO C++ forbids converting a string constant to ‘char*’ [-Werror=write-strings]
 static char *get_reg_name = STRINGIFY(UNW_OBJ(get_reg));
 ^~~~~~~~~~~~~~
/home/simark/src/binutils-gdb/gdb/common/preprocessor.h:28:25: note: in definition of macro ‘STRINGIFY_1’
 #define STRINGIFY_1(x) #x
                         ^
/home/simark/src/binutils-gdb/gdb/ia64-libunwind-tdep.c:114:29: note: in expansion of macro ‘STRINGIFY’
 static char *get_reg_name = STRINGIFY(UNW_OBJ(get_reg));
                             ^~~~~~~~~

gdb/ChangeLog:

	* ia64-tdep.c (get_reg_name, get_fpreg_name, get_saveloc_name,
	is_signal_frame_name, step_name, init_remote_name,
	create_addr_space_name, destroy_addr_space_name,
	search_unwind_table_name, find_dyn_list_name): Constify.