Revert "build.mk pass arch to the module compilation"

This reverts commit 7ec82f862a594136a1b4de800f4e5a44dc038076.

Glauber Costa writes:

  make image=redis-memonly now fails to build.

  the failure is:

  MAKE hiredis
  cd hiredis && make static
  make[6]: Entering directory `/home/glauber/osv-clean/apps/redis-memonly/upstream/redis-3.0.0-beta3/deps/hiredis'
  cc -std=c99 -pedantic -c -O3 -fPIC -fPIC -Wall -W -Wstrict-prototypes -Wwrite-strings -g -ggdb x64 net.c
  cc: error: x64: No such file or directory

  Looking at the history, it seems that it was introduced by
  7ec82f862a594136a1b4de800f4e5a44dc038076, and indeed, reverting it restores
  the build to normal.

Conflicts:
	build.mk