Clean up darwin cc toolchain

Remove the ability for Soong to build 32-bit darwin code. We've already
disabled this in Make, this is just removing the unused bits in Soong
and simplifing the toolchain config.

Test: m host
Test: build/soong/scripts/build-ndk-prebuilts.sh
Change-Id: I968c5d98bdf162297d639f7988918dadb7ba6e07

android/arch.go

@@ -201,7 +201,7 @@ var (
 	osArchTypeMap = map[OsType][]ArchType{
 		Linux:       []ArchType{X86, X86_64},
 		LinuxBionic: []ArchType{X86_64},
-		Darwin:      []ArchType{X86, X86_64},
+		Darwin:      []ArchType{X86_64},
 		Windows:     []ArchType{X86, X86_64},
 		Android:     []ArchType{Arm, Arm64, Mips, Mips64, X86, X86_64},
 	}