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_device.go b/cc/config/x86_device.go
index b9ce4af..15188cf 100644
--- a/cc/config/x86_device.go
+++ b/cc/config/x86_device.go
@@ -38,6 +38,8 @@
 		"-Wl,--hash-style=gnu",
 	}
 
+	x86Lldflags = ClangFilterUnknownLldflags(x86Ldflags)
+
 	x86ArchVariantCflags = map[string][]string{
 		"": []string{
 			"-march=prescott",
@@ -149,12 +151,14 @@
 
 	pctx.StaticVariable("X86Cflags", strings.Join(x86Cflags, " "))
 	pctx.StaticVariable("X86Ldflags", strings.Join(x86Ldflags, " "))
+	pctx.StaticVariable("X86Lldflags", strings.Join(x86Lldflags, " "))
 	pctx.StaticVariable("X86Cppflags", strings.Join(x86Cppflags, " "))
 	pctx.StaticVariable("X86IncludeFlags", bionicHeaders("x86"))
 
 	// Clang cflags
 	pctx.StaticVariable("X86ClangCflags", strings.Join(ClangFilterUnknownCflags(x86ClangCflags), " "))
 	pctx.StaticVariable("X86ClangLdflags", strings.Join(ClangFilterUnknownCflags(x86Ldflags), " "))
+	pctx.StaticVariable("X86ClangLldflags", strings.Join(ClangFilterUnknownCflags(x86Lldflags), " "))
 	pctx.StaticVariable("X86ClangCppflags", strings.Join(ClangFilterUnknownCflags(x86Cppflags), " "))
 
 	// Yasm flags
@@ -239,6 +243,10 @@
 	return "${config.X86Ldflags}"
 }
 
+func (t *toolchainX86) ClangLldflags() string {
+	return "${config.X86Lldflags}"
+}
+
 func (t *toolchainX86) YasmFlags() string {
 	return "${config.X86YasmFlags}"
 }