Commit 0d1a8d09 added a two step allocator for 64 bit systems. This
allocator mmaps a huge (1 TB) chunk of memory out of which it does
smaller allocations. On AArch64/Arm64 linux, this mmap was failing
due to the Arm64 Linux kernel parameter CONFIG_ARM64_VA_BITS
defaulting to 39 bits.

Therefore reducing the AArch64 value for MBLOCK_SPACE_SIZE to make
this allocation 1/4 TB while remaining 1 TB for other archs.

Reviewers: ezyang, austin, bgamari

Subscribers: thomie

Differential Revision: https://phabricator.haskell.org/D1171

GHC Trac Issues: #10682