Fix build for android

[cions]

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[CLAassistant]

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[bartlomieju]

LGTM, @piscisaureus any comments?

[piscisaureus]

LGTM

[Taknok]

Hi,
When trying to install the v8 from the fix-android, I have the following scenari.

1. The default install : the build.rs tries to download the *-android-*.a pre build binary from the github releases, which does not exist (yet) and fail.
2. The built from local. I am using termux. I already have gn, ninja and clang installed.
   A. The detection of clang fails and the build.rs downloads the chromium clang. The clang downloaded is a x86_64 arch, thus the execution fails on an ARM arch.
   B. Specifying the clang base path, the compilation starts but the #include <asm/errno.h> fails as it is not presents. Creating a sym link from asm-generic to asm. Trigger the fatal error: 'asm/sigcontext.h' file not found as it is not present in the generic ASM.
   C. Along with clang path, passing to the new env var CXXSTDLIB=/data/data/com.termux/files/usr/lib/libc++_shared.so also trigger the fatal error: 'asm/sigcontext.h' file not found

Does this PR only fix for x86 android or cross build ?

[cions]

I'm aiming for Termux on AArch64 Android.

This PR alone is not enough to build deno successfully.
See https://github.com/cions/termux-deno for build procedures.