[PARISC] head.S: section mismatch fixes

- move boot_args[] into the init section
- move $global$ into the read_mostly section
- fix the following two section mismatches:
WARNING: vmlinux.o(.text+0x9c): Section mismatch: reference to .init.text:start_kernel (between '$pgt_fill_loop' and '$is_pa20')
WARNING: vmlinux.o(.text+0xa0): Section mismatch: reference to .init.text:start_kernel (between '$pgt_fill_loop' and '$is_pa20')

Signed-off-by: Helge Deller <deller@gmx.de>
SIgned-off-by: Kyle McMartin <kyle@mcmartin.ca>