v8: cherry-pick uclibc build patch from upstream

Original commit log follows:

    Restore V8_LIBC_UCLIBC as a libc option.

    As uClibc defines __GLIBC__ in an attempt to look like glibc,
    V8_LIBC_GLIBC was true for uClibc as well. Checking for uClibc
    before glibc fixes this and restores the correct behavior.

    Review URL: https://codereview.chromium.org/1066573005

Fixes: https://github.com/nodejs/io.js/issues/1432
PR-URL: https://github.com/nodejs/io.js/pull/1974


Reviewed-By: Johan Bergström <bugs@bergstroem.nu>
Reviewed-By: Rod Vagg <rod@vagg.org>