Disable glibc tests on vxworks
Disable glibc tests on vxworks

VxWorks does not provide glibc, but we still need to test rustc on
VxWorks.

aidancully/rust,graydon/rust,graydon/rust,aidancully/rust,graydon/rust,aidancully/rust,graydon/rust,graydon/rust,aidancully/rust,aidancully/rust,aidancully/rust,graydon/rust