arc: Write correct "eret" value during register collection

In collect_register() function of arc-linux-tdep.c, the "eret"
(exception return) register value was not being reported correctly.
This patch fixes that.

Background:
When asked for the "pc" value, we have to update the "eret" register
with GDB's STOP_PC.  The "eret" instructs the kernel code where to
jump back when an instruction has stopped due to a breakpoint.  This
is how collect_register() was doing so:

--------------8<--------------
  if (regnum == gdbarch_pc_regnum (gdbarch))
    regnum = ARC_ERET_REGNUM;
  regcache->raw_collect (regnum, buf + arc_linux_core_reg_offsets[regnum]);
-------------->8--------------

Root cause:
Although this is using the correct offset (ERET register's), it is also
changing the REGNUM itself.  Therefore, raw_collect (regnum, ...) is
not reading from "pc" anymore.

v2:
- Fix a copy/paste issue as rightfully addressed by Tom [1].

[1]
https://sourceware.org/pipermail/gdb-patches/2020-November/173208.html

gdb/ChangeLog:

	* arc-linux-tdep.c (collect_register): Populate "eret" by
	"pc" value from the regcache when asked for "pc" value.

gdb/ChangeLog

@@ -1,3 +1,8 @@
+2020-12-04  Shahab Vahedi  <shahab@synopsys.com>
+
+	* arc-linux-tdep.c (collect_register): Populate "eret" by
+	"pc" value from the regcache when asked for "pc" value.
+
 2020-11-29  Hannes Domani  <ssbssa@yahoo.de>
 
 	PR tui/26973

gdb/arc-linux-tdep.c

@@ -319,8 +319,10 @@ static void
 collect_register (const struct regcache *regcache, struct gdbarch *gdbarch,
 		  int regnum, gdb_byte *buf)
 {
+  int offset;
+
   /* Skip non-existing registers.  */
-  if ((arc_linux_core_reg_offsets[regnum] == ARC_OFFSET_NO_REGISTER))
+  if (arc_linux_core_reg_offsets[regnum] == ARC_OFFSET_NO_REGISTER)
     return;
 
   /* The address where the execution has stopped is in pseudo-register
@@ -332,8 +334,10 @@ collect_register (const struct regcache *regcache, struct gdbarch *gdbarch,
      the program will continue at the address after the current instruction.
      */
   if (regnum == gdbarch_pc_regnum (gdbarch))
-    regnum = ARC_ERET_REGNUM;
+    offset = arc_linux_core_reg_offsets[ARC_ERET_REGNUM];
-  regcache->raw_collect (regnum, buf + arc_linux_core_reg_offsets[regnum]);
+  else
+    offset = arc_linux_core_reg_offsets[regnum];
+  regcache->raw_collect (regnum, buf + offset);
 }
 
 void