s390: simplify early program check handler

Due to historic reasons the base program check handler calls a
configurable function. Given that there is only the early program
check handler left, simplify the code by directly calling that
function.

The only other user was removed with commit d485235b00 ("s390:
assume diag308 set always works").

Also rename all functions and the asm file to reflect this.

Reviewed-by: Sven Schnelle <svens@linux.ibm.com>
Signed-off-by: Heiko Carstens <hca@linux.ibm.com>

arch/s390/include/asm/processor.h

@@ -304,12 +304,6 @@ static __always_inline void __noreturn disabled_wait(void)
 	while (1);
 }
 
-/*
- * Basic Program Check Handler.
- */
-extern void s390_base_pgm_handler(void);
-extern void (*s390_base_pgm_handler_fn)(struct pt_regs *regs);
-
 #define ARCH_LOW_ADDRESS_LIMIT	0x7fffffffUL
 
 extern int memcpy_real(void *, unsigned long, size_t);