Remove rlibc from the distribution

To make progress on #18585 we're paring down the distribution to theoretically
"only libstd", and this commit makes progress on this by removing the rlibc
crate from the distribution.

The crate has now been moved into an external cargo package located in the rust
lang organization [1]. This is a breaking change due to this removal, and
existing crates depending on `rlibc` should use the Cargo crate instead.

[1]: https://github.com/rust-lang/rlibc

[breaking-change]
cc #18585

Author: alexcrichton

mk/crates.mk

@@ -51,7 +51,7 @@
 
 TARGET_CRATES := libc std green native flate arena term \
                  serialize sync getopts collections test time rand \
-                 log regex graphviz core rbml rlibc alloc rustrt \
+                 log regex graphviz core rbml alloc rustrt \
                  unicode
 HOST_CRATES := syntax rustc rustdoc regex_macros fmt_macros \
 	       rustc_llvm rustc_back
@@ -60,7 +60,6 @@ TOOLS := compiletest rustdoc rustc
 
 DEPS_core :=
 DEPS_libc := core
-DEPS_rlibc := core
 DEPS_unicode := core
 DEPS_alloc := core libc native:jemalloc
 DEPS_rustrt := alloc core libc collections native:rustrt_native
@@ -104,7 +103,6 @@ TOOL_SOURCE_rustc := $(S)src/driver/driver.rs
 
 ONLY_RLIB_core := 1
 ONLY_RLIB_libc := 1
-ONLY_RLIB_rlibc := 1
 ONLY_RLIB_alloc := 1
 ONLY_RLIB_rand := 1
 ONLY_RLIB_collections := 1