Remove EXPLICIT_SIGN_EXTEND.

Remove EXPLICIT_SIGN_EXTEND.

* lisp.h (union Lisp_Object): Explicitly declare signedness of
bit-field.
(XINT): Remove variant for EXPLICIT_SIGN_EXTEND.
* m/alpha.h (EXPLICIT_SIGN_EXTEND): Don't define.
* m/amdx86-64.h (EXPLICIT_SIGN_EXTEND): Likewise.
* m/ia64.h (EXPLICIT_SIGN_EXTEND): Likewise.
* m/ibms390.h (EXPLICIT_SIGN_EXTEND): Likewise.
* m/ibms390x.h (EXPLICIT_SIGN_EXTEND): Likewise.
* m/iris4d.h (EXPLICIT_SIGN_EXTEND): Likewise.
* m/m68k.h (EXPLICIT_SIGN_EXTEND): Likewise.
* m/sparc.h (EXPLICIT_SIGN_EXTEND): Likewise.
* m/template.h (EXPLICIT_SIGN_EXTEND): Likewise.
* m/hp800.h: Remove file.
* m/mips.h: Remove file.

* CPP-DEFINES (EXPLICIT_SIGN_EXTEND): Remove.