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.

BUG=

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

Cr-Commit-Position: refs/heads/master@{#27806}