Update the vendored third-party dependencies for the mozjs-sys crate.
This picks up recent bug-fixes and reduces noise in unrelated runs
of 'mach vendor'.
The libc crate is also used by the rust url parser.
gcc 0.3.35 -> 0.3.40
libc 0.2.16 -> 0.2.18
libz-sys 1.0.6 -> 1.0.10
MozReview-Commit-ID: 5ri4nOtQQ1n