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

android/config.go

@@ -629,6 +629,10 @@ func (c *config) TargetOpenJDK9() bool {
 	return c.targetOpenJDK9
 }
 
+func (c *config) UseClangLld() bool {
+	return Bool(c.productVariables.UseClangLld)
+}
+
 func (c *config) ClangTidy() bool {
 	return Bool(c.productVariables.ClangTidy)
 }

android/variable.go

@@ -176,6 +176,8 @@ type productVariables struct {
 	OdmPath     *string `json:",omitempty"`
 	ProductPath *string `json:",omitempty"`
 
+	UseClangLld *bool `json:",omitempty"`
+
 	ClangTidy  *bool   `json:",omitempty"`
 	TidyChecks *string `json:",omitempty"`