The NDK build is using master-ndk, but the platform toolchains still
build from here directly.

This reverts commit 6f8740dd.

Change-Id: I6f131c2dd6c90ae12956d75422547348b356d8a4

Bug: http://b/21907643
Change-Id: Ie88401b21e0c4f6dfcebe08042f427a42c3b65f3

Change-Id: I7b6be0a637cd72cb5e2fa77ac4826be186b339ba

 * avr-libc is no longer (never was?) used.
 * mclinker is no longer used.
 * valgrind comes from the platform tree.

Change-Id: Ib0da75edf442252e4b33da98aef9964a713b10fb

Change-Id: Ief7f57035fd81c2d7f4a0cd0dfdb7cc19db3ae98

Change-Id: Ifcfcd38544db5fdfd29fe5968db5baf09c18599c

Change-Id: I09c12b4040288793047d4a1de769bcc3d9725aad

Change-Id: I028a66fb497981e7fa06cacc8d97e72c3f6e06d1

Change-Id: I2358fa04dfd37c7b20dcebbddf1f4eccf37aa129

Change-Id: I3b3f943243da34610e84bf987cd2f1546493f1de

Change-Id: I843ebb35be0ccd57a75ab2eaff513cbe3b143122