Refactor transform-o-to-* definitions.

Previously, there was one generic definition for each of the
transform-o-to-* functions in definitions.mk, and one target specific
one in each combo/TARGET_*.mk. The generic one was entirely unused,
and the target specific ones were all nearly identical.

Changing anything in these functions was tedious at best, and often
error prone. The differences between any 32-bit arch and its 64-bit
equivalent were restricted to the name of the linker, and the ARM and
MIPS definitions were identical. The few differences between ARM and
x86 looked to be compatibility for an old (ca. 2008) toolchain issue
with --gc-sections, and a bug (LDFLAGS coming first rather than
later).

To simplify things, I've moved the definitions for these out of
combo/TARGET_*.mk and back into definitions.mk. The differences
between ARM and x86 have been scrapped. Anything that really does
still need to be target specific can be handled as I have the linker:
add a TARGET_FOO variable to the given target and then add it to the
generic definition.

Change-Id: I54dc1bffc32ac39f27f0b87247dd6a6dbaf0b162