Merge pull request #2867 from alexrp/sgen-block-size-ppc

[sgen] Set ARCH_MIN_MS_BLOCK_SIZE to 64k on both ppc32 and ppc64.

This fixes a problem that was happening on Debian ppc32 build machines. Since
they are set up as a 32-bit userland on a 64-bit kernel, Mono would be compiled
with an ARCH_MIN_MS_BLOCK_SIZE value of 16k. However, at runtime, the system
page size would be 64k (due to the 64-bit kernel with PPC_64K_PAGES set).

Signed-off-by: `Alex Rønne Petersen <alexrp@xamarin.com>`