[gdb/build] Work around cgen odr violations

[gdb/build] Work around cgen odr violations

When building gdb with -flto -O2, I run into:
...
opcodes/mep-desc.h:250:14: warning: type 'cgen_operand_type' violates the \
  C++ One Definition Rule [-Wodr]
 typedef enum cgen_operand_type {
              ^
opcodes/or1k-desc.h:624:14: note: an enum with different value name is \
  defined in another translation unit
 typedef enum cgen_operand_type {
              ^
opcodes/mep-desc.h:212:14: warning: type 'cgen_hw_type' violates the C++ One \
  Definition Rule [-Wodr]
 typedef enum cgen_hw_type {
              ^
opcodes/or1k-desc.h:433:14: note: an enum with different value name is \
  defined in another translation unit
 typedef enum cgen_hw_type {
              ^
...

Fix this by making the conflicting type names unique, adding a target-specific
prefix using a define before the include:
...
 #define cgen_operand_type <target-name>_cgen_operand_type
 #define cgen_hw_type  <target-name>_cgen_hw_type
 #include "opcodes/<target-name>-desc.h"
...
and move those defines into a new file cgen-remap.h, similar to how that's
done for yacc in yy-remap.h.

Likewise for targets frv and lm32, the two other targets that include
opcodes/<target-name>-desc.h.

Likewise for more cgen symbols that I got the same warning for when using
-flto-partition=one.

A PR has been filed to take care of this in the opcodes dir instead (PR30758).

Tested on x86_64-linux.

Approved-By: Tom Tromey <tom@tromey.com>
PR build/30757
Bug: https://sourceware.org/bugzilla/show_bug.cgi?id=30757