Remove makefile comments about switching between JSC and V8

We no longer support JSC.

Also add a check for ARMv7, as this is the only version on which V8 is well
tested. Note that we no longer support ARMv5. See b/6015755.

Note that this was previously landed as
https://android-git.corp.google.com/g/#/c/162145, but broke the build, so was
reverted.

Bug: 5495373
Change-Id: Ibde55183ce8bc3b39e513f8b5d331219e793cf49
1 file changed