gdb: silence shellcheck warning SC2162 (use read -r) in gdbarch.sh

shellcheck reports:

    In gdbarch.sh line 53:
        while IFS='' read line
                     ^--^ SC2162: read without -r will mangle backslashes.

See the rationale at [1].  In our case, we actually want the backslashes
to be interpreted and removed.  Silence the warning using a directive.

[1] https://github.com/koalaman/shellcheck/wiki/SC2162

gdb/ChangeLog:

	* gdbarch.sh (do_read): Add shellcheck disable directive for
	warning SC2162.

gdb/ChangeLog

@@ -1,3 +1,8 @@
+2020-04-29  Simon Marchi  <simon.marchi@efficios.com>
+
+	* gdbarch.sh (do_read): Add shellcheck disable directive for
+	warning SC2162.
+
 2020-04-29  Simon Marchi  <simon.marchi@efficios.com>
 
 	* gdbarch.sh: Use ${foo:-} where shellcheck would report a

gdb/gdbarch.sh

@@ -50,6 +50,7 @@ do_read ()
     # On some SH's, 'read' trims leading and trailing whitespace by
     # default (e.g., bash), while on others (e.g., dash), it doesn't.
     # Set IFS to empty to disable the trimming everywhere.
+    # shellcheck disable=SC2162
     while IFS='' read line
     do
 	if test "${line}" = ""