fix osReserveHeapMemory block alignment

fix osReserveHeapMemory block alignment

Before the change osReserveHeapMemory() attempted
to allocate chunks of memory via osTryReserveHeapMemory()
not multiple of MBLOCK_SIZE in the following fallback code:

```
    if (at == NULL) {
        *len -= *len / 8;
```

and caused assertion failure:

```
$ make fulltest TEST=T11607 WAY=threaded1
T11607: internal error: ASSERTION FAILED: file rts/posix/OSMem.c, line 457
    (GHC version 8.7.20180716 for riscv64_unknown_linux)

```

The change applies alignment mask before each MBLOCK allocation attempt
and fixes WAY=threaded1 test failures on qemu-riscv64.

Signed-off-by: Sergei Trofimovich <slyfox@gentoo.org>
Test Plan: run 'make fulltest WAY=threaded1'

Reviewers: simonmar, bgamari, erikd

Reviewed By: simonmar

Subscribers: rwbarton, thomie, carter

Differential Revision: https://phabricator.haskell.org/D4982

Gbp-Pq: Name e175aaf6918bb2b497b83618dc4c270a0d231a1c.patch

bsymbolic-only-for-registerised

Gbp-Pq: Name bsymbolic-only-for-registerised.patch

llvm-arm-unknown-linux-gnueabi

===================================================================

Gbp-Pq: Name llvm-arm-unknown-linux-gnueabi.patch

use-stage1-binaries-for-install

Gbp-Pq: Name use-stage1-binaries-for-install.patch

x32-use-native-x86_64-insn

Gbp-Pq: Name x32-use-native-x86_64-insn.patch

buildpath-abi-stability

Forwarded to https://ghc.haskell.org/trac/ghc/ticket/10424

Gbp-Pq: Name buildpath-abi-stability.patch

no-missing-haddock-file-warning

Gbp-Pq: Name no-missing-haddock-file-warning

ARM-VFPv3D16

Gbp-Pq: Name ARM-VFPv3D16

use-debian-gen_contents_index

===================================================================

Gbp-Pq: Name use-debian-gen_contents_index

ghc (8.8.1+dfsg1+is+8.6.5+dfsg1-2) unstable; urgency=medium

  [ John Paul Adrian Glaubitz ]
  * Enable workaround to avoid memory exhaustation on alpha
  [ Gianfranco Costamagna ]
  * Source-only reupload now that the bootstrap has ended

[dgit import unpatched ghc 8.8.1+dfsg1+is+8.6.5+dfsg1-2]

Import ghc_8.8.1+dfsg1+is+8.6.5+dfsg1-2.debian.tar.xz

[dgit import tarball ghc 8.8.1+dfsg1+is+8.6.5+dfsg1-2 ghc_8.8.1+dfsg1+is+8.6.5+dfsg1-2.debian.tar.xz]

Import ghc_8.8.1+dfsg1+is+8.6.5+dfsg1.orig.tar.xz

[dgit import orig ghc_8.8.1+dfsg1+is+8.6.5+dfsg1.orig.tar.xz]