Fix absolute paths in clang debug output

As of the latest clang python wrapper, clang is being executed with an
absolute path. That means that clang-provided headers are written with
absolute paths in the debug info, making the builds non-reproducible
(especially with ccache).

So keep our current PWD=/proc/self/pwd -fdebug-prefix-map=/proc/self/cwd=,
but add a new -fdebug-prefix-map=$PWD/= to rewrite these absolute paths.

Bug: 36463318
Test: m -j libc adb; grep for path names
Test: spot check objdump -W before/after, see only expected diffs.
Change-Id: Ic2cdfca051d3c38b753fd5c1fcbeaca8bafc29ae