nss: fix building for aarch32 on aarch64
nss: fix building for aarch32 on aarch64

There are two ways to build a package for aarch32 on an aarch64 machine:
either by cross compiling as normal, or by adding armv6l/armv7l to
extraPlatforms and doing a non-cross compile.

Previously, NSS failed to build with both methods: when using
extraPlatforms, things failed because NSS includes an armv8-specific
file (presumably based on the result of uname); when cross compiling,
NSS's build system expects to receive an architecture name of arm (not
armv6l or whatever), so was failing to include some arch-specific code
and failed with a linker error.

This commit fixes those things by a) always passing the arch, even when
not cross-compiling, and b) special-casing aarch32 to always pass in an
arch of arm.

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