Add missing pieces of ARMv9.2-A support.

aosp/3255916 didn't add all of the required infrastructure.
Refactor it to be more like x86_64 so it requires less
boilerplate code.

Bug: 364383722
Test: presubmit
Change-Id: Idc5279ffc5fe964bea9f98873c01468944d90091
diff --git a/cc/config/arm64_linux_host.go b/cc/config/arm64_linux_host.go
index 438e0e6..a19b0ed 100644
--- a/cc/config/arm64_linux_host.go
+++ b/cc/config/arm64_linux_host.go
@@ -87,8 +87,8 @@
 }
 
 func linuxBionicArm64ToolchainFactory(arch android.Arch) Toolchain {
-	archVariant := "armv8-a" // for host, default to armv8-a
-	toolchainCflags := []string{arm64ArchVariantCflagsVar[archVariant]}
+	// for host, default to armv8-a
+	toolchainCflags := []string{"-march=armv8-a"}
 
 	// We don't specify CPU architecture for host. Conservatively assume
 	// the host CPU needs the fix