Introduce rootfs for cross-compilation

[hqueue]

skip ci please

Introduce rootfs for cross compilation. I've added scripts for ARM and ARM softfp.
Tested it with sudo ./cross/build-rootfs.sh arm and sudo ./cross/build-rootfs.sh arm-softfp as documented in https://github.com/dotnet/coreclr/blob/master/Documentation/building/cross-building.md to keep consistency with CoreCLR and CoreFX.

Basically the code has been imported from CoreCLR and remove x86 and arm64.
rootfs is exactly same with CoreCLR, i.e. we can also build CoreCLR with this rootfs too.
Later we can refine rootfs to reduce size only with required packages if necessary.
BTW CoreFX has a different configuration.

[dnfclas]

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[hqueue]

CC @schellap @ramarag @janvorli @gkhanna79 @kouvel PTAL

Related issue: #729

[gkhanna79]

Basically the code has been imported from CoreCLR and remove x86 and arm64

I was thinking about this. Since there is work going on to bring Linux for x86 (https://github.com/dotnet/coreclr/issues/8188), you may just want to keep the x86 support as well (but do not need to test it).

Aside from this, LGTM.

[hqueue]

CC @gkhanna79 @schellap @ramarag @janvorli I've added x86 rootfs as suggested.

[gkhanna79]

LGTM - assuming you have validated it, right?

[hqueue]

@gkhanna79 Yes. For ARM, we built corehost for ARM after appling this PR #747 and PR #749 together and rootfs for x86 is built with this PR #747.