Fix cast types for opencl

Fix cast types for opencl

The bitshift tests for opencl have these failures:

print /x (signed char) 0x0f << 8
No type named signed char.
(gdb) FAIL: gdb.base/bitshift.exp: lang=opencl: 8-bit, promoted: print /x (signed char) 0x0f << 8
print (signed char) 0x0f << 8
No type named signed char.
(gdb) FAIL: gdb.base/bitshift.exp: lang=opencl: 8-bit, promoted: print (signed char) 0x0f << 8

Apparently opencl doesn't have the 'signed' modifier for types, only
the 'unsigned' modifier.
Even 'char' is guaranteed to be signed if no modifier is used, so
this changes the casts to match this logic.

Approved-By: Tom Tromey <tom@tromey.com>