Add USE_CLANG_LLD and use_clang_lld.

* USE_CLANG_LLD is unedefined in current builds.
* When USE_CLANG_LLD is defined to 'true' or '1',
  use clang's lld instead of ld or ld.gold.
* When lld is enabled:
  * ld-only flags are not passed to 'lld'.
  * location_packer is disabled.
  * Use new lld's --pack-dyn-relocs=android.
* When lld does not work:
  * In Android.mk files use LOCAL_USE_CLANG_LLD := false.
  * In Android.bp files use use_clang_lld: false.
* Only arm, arm64, x86, and x86_64_devices have LLD flags;
  all other hosts and targets do not call lld yet.

Bug: 73768157
Test: make checkbuild and boot
Change-Id: I06b8a1e868a600997a7e70fe05c299d751d23d5f
diff --git a/cc/config/x86_64_device.go b/cc/config/x86_64_device.go
index 12f3e6f..ba03094 100644
--- a/cc/config/x86_64_device.go
+++ b/cc/config/x86_64_device.go
@@ -32,6 +32,8 @@
 		"-Wl,--hash-style=gnu",
 	}
 
+	x86_64Lldflags = ClangFilterUnknownLldflags(x86_64Ldflags)
+
 	x86_64ArchVariantCflags = map[string][]string{
 		"": []string{
 			"-march=x86-64",
@@ -125,12 +127,14 @@
 
 	pctx.StaticVariable("X86_64Cflags", strings.Join(x86_64Cflags, " "))
 	pctx.StaticVariable("X86_64Ldflags", strings.Join(x86_64Ldflags, " "))
+	pctx.StaticVariable("X86_64Lldflags", strings.Join(x86_64Lldflags, " "))
 	pctx.StaticVariable("X86_64Cppflags", strings.Join(x86_64Cppflags, " "))
 	pctx.StaticVariable("X86_64IncludeFlags", bionicHeaders("x86"))
 
 	// Clang cflags
 	pctx.StaticVariable("X86_64ClangCflags", strings.Join(ClangFilterUnknownCflags(x86_64Cflags), " "))
 	pctx.StaticVariable("X86_64ClangLdflags", strings.Join(ClangFilterUnknownCflags(x86_64Ldflags), " "))
+	pctx.StaticVariable("X86_64ClangLldflags", strings.Join(ClangFilterUnknownCflags(x86_64Lldflags), " "))
 	pctx.StaticVariable("X86_64ClangCppflags", strings.Join(ClangFilterUnknownCflags(x86_64Cppflags), " "))
 
 	// Yasm flags
@@ -215,6 +219,10 @@
 	return "${config.X86_64Ldflags}"
 }
 
+func (t *toolchainX86_64) ClangLldflags() string {
+	return "${config.X86_64Lldflags}"
+}
+
 func (t *toolchainX86_64) YasmFlags() string {
 	return "${config.X86_64YasmFlags}"
 }