Stop encoding absolute paths in symbols

GCC & Clang encode the current working directory into the symbol files.
Even if the binaries are stripped before being installed, the build-id
hash will be different. This means that two different checkouts will
produce different installed binaries, even if the sources are identical.

To prevent this, set PWD=/proc/self/cwd, so that it replaces the
absolute path with the static path /proc/self/cwd. With GCC, use
-fdebug-prefix-map to remove that static path. Our clang does not yet
support this flag, but it was merged in r250094.

This does change the behavior when using gdb -- you now need to be in
$ANDROID_BUILD_TOP before gdb will load the sources.

Bug: 24204119
Change-Id: I792d65b042b7a3fdc78d567c5a6e93cdf0141b9b