parisc: Switch to generic COMPAT_BINFMT_ELF

Drop our own compat binfmt implementation in
arch/parisc/kernel/binfmt_elf32.c in favour of the generic
implementation with CONFIG_COMPAT_BINFMT_ELF.

While cleaning up the dependencies, I noticed that ELF_PLATFORM was strangely
defined: On a 32-bit kernel, it was defined to "PARISC", while when running in
compat mode on a 64-bit kernel it was defined to "PARISC32". Since it doesn't
seem to be used in glibc yet, it's now defined in both cases to "PARISC". In
any case, it can be distinguished because it's either a 32-bit or a 64-bit ELF
file.

Signed-off-by: Helge Deller <deller@gmx.de>