[Driver] Search for compiler-rt libraries in sysroot/lib if not found in the resource directory

This patch improves support for embedded Linux toolchains with multiple
sysroots, each containing different built-in libraries. If compiler-rt
libraries are not found in the resource directory, the Clang driver will
fallback to searching in the sysroot/lib directory.

This change preserves the standard behavior for regular Linux toolchains
while allowing greater flexibility in toolchains with multiple sysroots.

Author: Kostiantyn Lazukin

clang/lib/Driver/ToolChains/Linux.cpp

@@ -354,6 +354,8 @@ Linux::Linux(const Driver &D, const llvm::Triple &Triple, const ArgList &Args)
 
   addPathIfExists(D, concat(SysRoot, "/lib"), Paths);
   addPathIfExists(D, concat(SysRoot, "/usr/lib"), Paths);
+
+  addPathIfExists(D, concat(SysRoot, "/lib"), getLibraryPaths());
 }
 
 ToolChain::RuntimeLibType Linux::GetDefaultRuntimeLibType() const {

clang/test/Driver/Inputs/musl_sysroot_with_builtins/lib/Scrt1.o

@@ -0,0 +1,11 @@
+// RUN: %clang -### %s 2>&1 \
+// RUN:     --target=aarch64-unknown-linux-musl \
+// RUN:     -ccc-install-dir %S/Inputs/basic_linux_tree/usr/bin \
+// RUN:     -resource-dir=%S/Inputs/resource_dir_empty \
+// RUN:     --sysroot=%S/Inputs/musl_sysroot_with_builtins \
+// RUN:     -static \
+// RUN:     -rtlib=compiler-rt \
+// RUN:   | FileCheck %s
+
+// CHECK-DAG: "--sysroot={{[^"]+}}/Inputs/musl_sysroot_with_builtins"
+// CHECK-DAG: "{{[^"]+}}/Inputs/musl_sysroot_with_builtins/lib/libclang_rt.builtins.a"