RISC-V nommu support v6

Hi all,

below is a series to support nommu mode on RISC-V.  For now this series
just works under qemu with the qemu-virt platform, but Damien has also
been able to get kernel based on this tree with additional driver hacks
to work on the Kendryte KD210, but that will take a while to cleanup
an upstream.

A git tree is available here:

    git://git.infradead.org/users/hch/riscv.git riscv-nommu.6

Gitweb:

    http://git.infradead.org/users/hch/riscv.git/shortlog/ref...

I've also pushed out a builtroot branch that can build a RISC-V nommu
root filesystem here:

   git://git.infradead.org/users/hch/buildroot.git riscv-nommu.2

Gitweb:

   http://git.infradead.org/users/hch/buildroot.git/shortlog...


Changes since v5:
 - rebased to Linux 5.4-rc5 
 - fix up a newly sneaked in use of ->sepc in the perf callchain code
 - fix out of tree builds with the generated loader.lds
 - replace the plic context hack with a cleaner solution

Changes since v4:
 - rebased to 5.4-rc + latest riscv fixes
 - clean up do_trap_break
 - fix an SR_XPIE issue (Paul Walmsley)
 - use the symbolic PAGE_OFFSET value in the flat loader
   (Aurabindo Jayamohanan)

Changes since v3:
 - improve a few commit message
 - cleanup riscv_cpuid_to_hartid_mask
 - cleanup the timer handling
 - cleanup the IPI handling a little more
 - renamed CONFIG_M_MODE to CONFIG_RISCV_M_MODE
 - split out CONFIG_RISCV_SBI to make some of the ifdefs more obbious
 - use IS_ENABLED wherever possible instead of if ifdefs to make the
   code more readable

Changes since v2:
 - rebased to 5.3-rc
 - remove the EFI image header for nommu builds
 - set ARCH_SLAB_MINALIGN to ensure stack alignment in the flat binary
   loader
 - minor comment improvement
 - use #defines for more CSRs

Changes since v1:
 - fixes so that a kernel with this series still work on builds with an
   IOMMU
 - small clint cleanups
 - the binfmt_flat base and buildroot now don't put arguments on the stack