gn build: Bump Android API level to 29, and NDK to r21.

[pcc]

Summary

These are both necessary in order to use ELF TLS, which is needed
by the scudo unit tests.

[merge_guards_bot]

Build result: pass - 60520 tests passed, 0 failed and 726 were skipped.

Log files: console-log.txt, CMakeCache.txt

[phosek]

LGTM

llvm/utils/gn/build/toolchain/compiler.gni

 declare_args() {
   # Whether to use goma (https://chromium.googlesource.com/infra/goma/client/)
   use_goma = false
 
   # Set this to a clang build directory. If set, that clang is used as compiler.
   # goma only works with compiler binaries it knows about, so useful both for
   # using a goma-approved compiler and for compiling clang with a locally-built
   # clang in a different build directory.
   # On Windows, setting this also causes lld-link to be used as linker.
   # Example value: getenv("HOME") + "/src/llvm-build/Release+Asserts"
   clang_base_path = ""
 
-  # Set this to the path to Android NDK r19. If set, cross compilation targeting
+  # Set this to the path to Android NDK r21. If set, cross compilation targeting
   # Android will be enabled.
   android_ndk_path = ""
 }
 
 declare_args() {
   # Set if the host compiler is clang.  On by default on Mac or if
   # clang_base_path is set.
   is_clang = host_os == "mac" || clang_base_path != ""
 
   # Set this to true to link with LLD instead of the default linker.
   use_lld = clang_base_path != "" && host_os != "mac"
 }

llvm/utils/gn/secondary/llvm/triples.gni

 if (current_cpu == "x86") {
   if (current_os == "linux") {
     llvm_current_triple = "i386-unknown-linux-gnu"
   }
 } else if (current_cpu == "x64") {
   if (current_os == "freebsd") {
     llvm_current_triple = "x86_64-unknown-freebsd"
   } else if (current_os == "linux") {
     llvm_current_triple = "x86_64-unknown-linux-gnu"
   } else if (current_os == "mac") {
     llvm_current_triple = "x86_64-apple-darwin"
   } else if (current_os == "win") {
     llvm_current_triple = "x86_64-pc-windows-msvc"
   }
 } else if (current_cpu == "arm") {
   if (current_os == "android") {
-    llvm_current_triple = "arm-linux-androideabi"
+    llvm_current_triple = "arm-linux-androideabi29"
   }
 } else if (current_cpu == "arm64") {
   if (current_os == "android") {
-    llvm_current_triple = "aarch64-linux-android21"
+    llvm_current_triple = "aarch64-linux-android29"
   }
 } else if (current_cpu == "ppc64") {
   if (current_os == "linux") {
     llvm_current_triple = "powerpc64le-unknown-linux-gnu"
   }
 }
 
 if (!defined(llvm_current_triple)) {
   assert(false, "unimplemented cpu/os " + current_cpu + "/" + current_os)
 }
 
 declare_args() {
   # The default target triple.
   llvm_target_triple = llvm_current_triple
 }