Use -march=armv9-a for Armv9-A arch variant

Additional context from krzysio@google.com in aosp/3544179: Currently, TARGET_ARCH_VARIANT := armv9-a actually uses armv8.2-a+dotprod, which seems to have been a workaround for toolchain issues that have since been fixed.  Right now only one proto board uses this TARGET_ARCH_VARIANT internally, so it seems reasonable to correct this.

Test: llvm-objdump -d
Test: presubmit
Change-Id: I99db407eb96d82d189378fd5abd4cc80ab802f20