This updates the following third-party dependencies of mozjs_sys:
gcc v0.3.40 -> v0.3.42
libc v0.2.18 -> v0.2.20
libz-sys v1.0.10 -> v1.0.12
pkg-config v0.3.8 -> v0.3.9
Since libc is updated, we also need to update the gkrust lockfiles to use the
new version, because leaving it at 0.2.18 will result in improper vendoring of
the crates (see bug 1336528). None of the other mozjs_sys crates are shared by
gkrust.
MozReview-Commit-ID: 5FHELF8YKD0
85aad79ed8