[MIPS] Unite mipsel and mips64el 4.9 gcc toolchains

Use 4.9 mips64el toolchain for both 64- and 32-bit builds.
Tell ld when 32-bit links are required.
Override 4.9's changed defaults for mips floating point
register use, to get same assembler rules as 4.8 and earlier.

Also: drop unused  soft-fp build targets, cleanout redundant
compiler options, and remove extraneous Android.mk file.

Change-Id: I86f1075266349edb2b08a7709b9f5472d8cfda32