allarch: Generate same package for MIPS and non-MIPS targets

LINKER_HASH_STYLE differs between MIPS and non-MIPS targets. This means
that LDFLAGS differs too. LDFLAGS is exported so it influences all task
hashes. Unfortunately this means that packages with architecture "all"
differ depending on whether they are built for a MIPS or non-MIPS target.
This causes a lot of unnecessary churn in the ipk/all directory when
switching build targets.

The simplest way to fix this is to ensure that LDFLAGS stays the same for
architecture "all" packages by clearing it. It shouldn't being used by such
packages anyway.

Signed-off-by: Mike Crowe <mac@mcrowe.com>
Signed-off-by: Richard Purdie <richard.purdie@linuxfoundation.org>