Bug 1088683: Rename TARGET_TOOLS_PREFIX to GECKO_TOOLS_PREFIX.

TARGET_TOOLS_PREFIX is now stored as GECKO_TOOLS_PREFIX in .var.profile
file [1]. This broke profile-symbolicate.

Rename the environment variable in profile-symbolicate to get things
working again.

[1] https://github.com/mozilla-b2g/gonk-misc/commit/b183966d7084f992ec5ff9ea6a486e20dd1fc6d6