Make rust builds hermetic

Rust builds were picking up the crt objects and system libraries like
libc.so from /usr/lib/x86_64-linux-gnu/Scrt1.o and
/lib/x86_64-linux-gnu/libc.so.6.  Pass --sysroot to the linker to
point it to the glibc prebuilts.

Test: manual
Change-Id: I6540df8aef0e5c2258de77456d18a6052db627d4

rust/config/x86_linux_host.go

@@ -26,6 +26,7 @@ var (
 		"-B${cc_config.ClangBin}",
 		"-fuse-ld=lld",
 		"-Wl,--undefined-version",
+		"--sysroot ${cc_config.LinuxGccRoot}/sysroot",
 	}
 	linuxX86Rustflags   = []string{}
 	linuxX86Linkflags   = []string{}