Add linux_musl arm+arm64

Add toolchains to support cross compiling to aarch64-linux-musl and
arm-linux-musleabihf.

Bug: 236052820
Test: build arm and arm64 musl sysroots
Change-Id: I47a9322929baff2492c6e8db989ece01fcbeb133

bp2build/cc_object_conversion_test.go

@@ -414,6 +414,7 @@ func TestCcObjectSelectOnLinuxAndBionicArchs(t *testing.T) {
             "bionic_arm64.cpp",
         ],
         "//build/bazel/platforms/os_arch:linux_glibc_x86": ["linux_x86.cpp"],
+        "//build/bazel/platforms/os_arch:linux_musl_arm64": ["linux_arm64.cpp"],
         "//build/bazel/platforms/os_arch:linux_musl_x86": ["linux_x86.cpp"],
         "//conditions:default": [],
     })`,