s390: unify linker symbols usage

Common code defines linker symbols which denote sections start/end in
a form of char []. Referencing those symbols as _symbol or &_symbol
yields the same result, but "_symbol" form is more widespread across
newly written code. Convert s390 specific code to this style.

Also removes unused _text symbol definition in boot/compressed/misc.c.

Signed-off-by: Vasily Gorbik <gor@linux.vnet.ibm.com>
Acked-by: Heiko Carstens <heiko.carstens@de.ibm.com>
Signed-off-by: Martin Schwidefsky <schwidefsky@de.ibm.com>