Auto merge of #26296 - aidanhs:aphs-fix-musl-make-install, r=alexcrichton

musl only creates rlib files for stdlib linking so we need to ignore the `CFG_LIB_GLOB_` setting, otherwise we an error:
```
$ make --debug VERBOSE=1 dist-tar-bins
[...]
            Successfully remade target file `prepare-target-x86_64-unknown-linux-gnu-host-x86_64-unknown-linux-gnu-2-dir-x86_64-unknown-linux-gnu'.
             File `prepare-target-x86_64-unknown-linux-musl-host-x86_64-unknown-linux-gnu-2-dir-x86_64-unknown-linux-gnu' does not exist.
            Must remake target `prepare-target-x86_64-unknown-linux-musl-host-x86_64-unknown-linux-gnu-2-dir-x86_64-unknown-linux-gnu'.
umask 022 && mkdir -p tmp/dist/rustc-1.2.0-dev-x86_64-unknown-linux-gnu-image/lib/rustlib/x86_64-unknown-linux-musl/lib
umask 022 && mkdir -p tmp/dist/rustc-1.2.0-dev-x86_64-unknown-linux-gnu-image/lib/rustlib/x86_64-unknown-linux-gnu/bin
LIB_NAME="liblibc-d8ace771.rlib"; MATCHES=""; if [ -n "$MATCHES" ]; then echo "warning: one or libraries matching Rust library 'liblibc-*.rlib'" && echo "  (other than '$LIB_NAME' itself) alre
ady present"     && echo "  at destination tmp/dist/rustc-1.2.0-dev-x86_64-unknown-linux-gnu-image/lib/rustlib/x86_64-unknown-linux-musl/lib:"      && echo $MATCHES ; fi
install -m644 `ls -drt1 x86_64-unknown-linux-gnu/stage2/lib/rustlib/x86_64-unknown-linux-musl/lib/liblibc-*.rlib` tmp/dist/rustc-1.2.0-dev-x86_64-unknown-linux-gnu-image/lib/rustlib/x86_64-unk
nown-linux-musl/lib/
LIB_NAME=""; MATCHES=""; if [ -n "$MATCHES" ]; then echo "warning: one or libraries matching Rust library 'libstd-*.so'" && echo "  (other than '$LIB_NAME' itself) already present"     && echo
 "  at destination tmp/dist/rustc-1.2.0-dev-x86_64-unknown-linux-gnu-image/lib/rustlib/x86_64-unknown-linux-musl/lib:"      && echo $MATCHES ; fi
install -m644 `ls -drt1 x86_64-unknown-linux-gnu/stage2/lib/rustlib/x86_64-unknown-linux-musl/lib/libstd-*.so` tmp/dist/rustc-1.2.0-dev-x86_64-unknown-linux-gnu-image/lib/rustlib/x86_64-unknow
n-linux-musl/lib/
ls: cannot access x86_64-unknown-linux-gnu/stage2/lib/rustlib/x86_64-unknown-linux-musl/lib/libstd-*.so: No such file or directory
install: missing destination file operand after ‘tmp/dist/rustc-1.2.0-dev-x86_64-unknown-linux-gnu-image/lib/rustlib/x86_64-unknown-linux-musl/lib/’
Try 'install --help' for more information.
make: *** [prepare-target-x86_64-unknown-linux-musl-host-x86_64-unknown-linux-gnu-2-dir-x86_64-unknown-linux-gnu] Error 1
```

`CFG_INSTALL_ONLY_RLIB_` is provided for this reason and fixes `make install` and `make dist`.

Author: bors

mk/cfg/x86_64-unknown-linux-musl.mk

@@ -3,6 +3,7 @@ CC_x86_64-unknown-linux-musl=$(CFG_MUSL_ROOT)/bin/musl-gcc
 CXX_x86_64-unknown-linux-musl=notaprogram
 CPP_x86_64-unknown-linux-musl=$(CFG_MUSL_ROOT)/bin/musl-gcc -E
 AR_x86_64-unknown-linux-musl=$(AR)
+CFG_INSTALL_ONLY_RLIB_x86_64-unknown-linux-musl = 1
 CFG_LIB_NAME_x86_64-unknown-linux-musl=lib$(1).so
 CFG_STATIC_LIB_NAME_x86_64-unknown-linux-musl=lib$(1).a
 CFG_LIB_GLOB_x86_64-unknown-linux-musl=lib$(1)-*.so