Update to Android NDK r13.

Update to Android NDK r13.

The libc++ include paths have changed very slightly.  I've left GN compatible with both older r12 NDKs and the new r13 to smooth the transition.

The libc++ in r13 depends on long-double math.h functions (cosl, atanl, etc.) only available in Android API v21 (Lollipop) and up.  That's what the 64-bit bots were already on, so we just pull the 32-bit bots up to the same target.  Conveniently, the oldest bots we have (N7 and N10) are on Lollipop.

The r13 MIPS64 sysroots are a little weird... /usr/include and /usr/lib64 but no /usr/lib.  That'd be fine---we only want 64-bit builds---but Clang searches for /usr/lib64 via its path to /usr/lib, and without at least an empty /usr/lib, it can't find /usr/lib64.  So you'll see a special mips64el section in the GN config where we do this all manually (other platforms pick this all up correctly from --sysroot).  I've chosen to do this rather than fix it up in the asset create.py scripts so that we stay compatible with vanilla NDKs, which is convenient for developers.

CQ_INCLUDE_TRYBOTS=master.client.skia.compile:Build-Mac-Clang-arm64-Debug-GN_Android-Trybot,Build-Ubuntu-Clang-arm-Debug-GN_Android-Trybot,Build-Ubuntu-Clang-arm-Release-GN_Android-Trybot,Build-Ubuntu-Clang-arm64-Debug-GN_Android-Trybot,Build-Ubuntu-Clang-arm64-Debug-GN_Android_FrameworkDefs-Trybot,Build-Ubuntu-Clang-arm64-Debug-GN_Android_Vulkan-Trybot,Build-Ubuntu-Clang-arm64-Release-GN_Android-Trybot,Build-Ubuntu-Clang-mips64el-Debug-GN_Android-Trybot,Build-Ubuntu-Clang-mips64el-Release-GN_Android-Trybot,Build-Ubuntu-Clang-mips64el-Release-GN_Android-Trybot,Build-Ubuntu-Clang-mipsel-Debug-GN_Android-Trybot,Build-Ubuntu-Clang-mipsel-Release-GN_Android-Trybot,Build-Ubuntu-Clang-x64-Debug-GN_Android-Trybot,Build-Ubuntu-Clang-x64-Release-GN_Android-Trybot,Build-Ubuntu-Clang-x86-Debug-GN_Android-Trybot,Build-Ubuntu-Clang-x86-Release-GN_Android-Trybot

BUG=skia:

GOLD_TRYBOT_URL= https://gold.skia.org/search?issue=3180

Change-Id: I6f3b5d9411ded0ee49c1099490f41fa86a8736f8
Reviewed-on: https://skia-review.googlesource.com/3180
Reviewed-by: Eric Boren <borenet@google.com>
Commit-Queue: Mike Klein <mtklein@chromium.org>