support armv7

[somombo]

Related to denoland/deno#2295
(See also denoland/deno#1846)

@piscisaureus following up on #265 (comment), I would appreciate your help and guidance on this.

/cc @afinch7 @liushuyu

[somombo]

So far, I have proceeded mostly by analogy, copying and altering what was done for arm64.

However, on my fork, I can't seem to get many of the tests in test_api.rs to pass. All other tests that are not in that particular file are passing ok.

[somombo]

Here is my latest build with the error I'm reproducibly getting:
https://github.com/somombo/rusty_v8/runs/3190160275?check_suite_focus=true

As you can see, I'm able to cargo build, cargo run hello world example, pass all tests other than those in test_api.rs all just fine.

However, when I try to run many of the tests in test_api.rs I get the following error

Fatal error in ../../../../v8/src/heap/read-only-spaces.cc, line 69
Check failed: read_only_blob_checksum_ == snapshot_checksum (<unprintable> vs. 4042857134).

My crud research so far, is pointing to that the error is because I set environment GN_ARGS='v8_enable_shared_ro_heap=true'
Which it told me that I needed to do for target armv7 or else it wouldn't even build let alone test.
(to be precise, it said I need to set either v8_enable_shared_ro_heap or v8_enable_pointer_compression and I found that the latter is not supported on this machine type)

/cc @nazrhom, @nghiant2710

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