[android-toolchain] Remove use of $(MAKEFLAGS)

[jonpryor]

An attempt to bump external/mono/ resulted in a
build failure on a macOS machine:

Executing: make MSBUILD_ARGS=/p:AutoProvision=True/ /p:AutoProvisionUsesSudo=True V=1 MXE_TARGETS="i686-w64-mingw32.static" gcc cmake zlib pt hreads dlfcn-win32 mman-win32 PREFIX="/Users/builder/android-toolchain/mxe"
make[1]: *** No rule to make target `/p:AutoProvisionUsesSudo=True'.  Stop.

(This happens on a macOS machine that hasn't previously
built+installed MXE which needs MXE in order to generate Windows
binaries, so auto-builds MXE...)

This error is due to the interraction of three separate choices:

1. Jenkins is running:

   make prepare jenkins V=1 MSBUILD_ARGS="/p:AutoProvision=True /p:AutoProvisionUsesSudo=True"
   

2. xbuild replaces \ with /.

3. The _CreateMxeToolchains target uses make $(MAKEFLAGS).

Background: what is $(MAKEFLAGS)? $(MAKEFLAGS) is set by
make(1) and contains the command-line flags to make.
It's value is shown elsewhere in the log file:

MAKEFLAGS = MSBUILD_ARGS=/p:AutoProvision=True\ /p:AutoProvisionUsesSudo=True V=1

Note that $(MAKEFLAGS) contains a backslash: this is because of the
original command, which provides a space-containing value for
$(MSBUILD_ARGS):

MSBUILD_ARGS="/p:AutoProvision=True /p:AutoProvisionUsesSudo=True"

Instead of preserving the quotes, make instead replaces (space)
with \ (backslash-space), which is perfectly valid:

# MAKEFLAGS set "as if" we executed:
make prepare jenkins V=1 MSBUILD_ARGS=/p:AutoProvision=True\ /p:AutoProvisionUsesSudo=True

The problem is when xbuild enters the picture: xbuild can't
differentiate between paths and...anything else, so for simplicity and
sanity it always replaces \ with the directory-separtor-char,
which is / on macOS/Linux.

Unfortunately, this completely changes the semantics of the embedded
MSBUILD_ARGS value within $(MAKEFLAGS): replacing \ with /
causes the /p:AutoProvisionUsesSudo=True to be treated as target by
make, and that target doesn't exist.

In theory, xbuild could be fixed to address this. In practice,
xbuild isn't getting any future work. msbuild is getting more
work, but this "corner case" is likely sufficiently complicated that
it might not ever get fixed.

Thus, the simple fix: Don't Do That™: remove use of $(MAKEFLAGS).
It isn't needed in this invocation.