RISC-V: Always compile mm/init.c with cmodel=medany and notrace

The Linux RISC-V 32bit kernel is broken after we moved setup_vm() from
kernel/setup.c to mm/init.c because Linux RISC-V 32bit kernel by default
uses cmodel=medlow which results in a non-position-independent setup_vm().

This patch fixes Linux RISC-V 32bit kernel booting by:
1. Forcing cmodel=medany for mm/init.c
2. Moving remaing MM-related stuff va_pa_offset, pfn_base and
   empty_zero_page from kernel/setup.c to mm/init.c

Further, the setup_vm() cannot handle GCC instrumentation for FTRACE so
we disable it for mm/init.c by not using "-pg" compiler flag.

Fixes: 6f1e9e94

 ("RISC-V: Move setup_vm() to mm/init.c")
Suggested-by: Christoph Hellwig <hch@lst.de>
Suggested-by: Mike Rapoport <rppt@linux.ibm.com>
Signed-off-by: Anup Patel <anup.patel@wdc.com>
Reviewed-by: Mike Rapoport <rppt@linux.ibm.com>
Reviewed-by: Christoph Hellwig <hch@lst.de>
Signed-off-by: Palmer Dabbelt <palmer@sifive.com>