More aggressive remapping

[mkeeter]

Looking at our strings, I noticed two things:

• /toolchain remaps were always of the form /toolchain/lib/rustlib/src/rust/library/core/src
• There were a bunch of strings of the form /hubris/target/thumbv7em-none-eabihf/release/build, which are files generated from build.rs scripts

This PR remaps those strings more aggressively, to /rustlib and /hubris/build respectively.

This saves 17920 bytes of flash on the cosmo-b-dev image:

auxflash            15936   -> 15864
control_plane_agent 114488  -> 113928
cosmo_seq           43684   -> 38304 (!)
dump_agent          16860   -> 16712
fmc_demo            10312   -> 10236
hash_driver         6984    -> 6912
hf                  34704   -> 34552
hiffy               22568   -> 22416
host_sp_comms       68860   -> 68484
i2c_driver          12192   -> 12156
idle                88      -> 88
ignition_flash      7488    -> 7452
jefe                9184    -> 9184
kernel              32768   -> 32768
net                 74872   -> 74648
packrat             22868   -> 22800
power               34920   -> 34768
rng_driver          5800    -> 5724
sensor              8280    -> 8244
snitch              3340    -> 3340
spartan7_loader     12924   -> 12812
spd                 6788    -> 6712
spi2_driver         11020   -> 10948
spi3_driver         10544   -> 10472
sprot               45252   -> 45172
sys                 2720    -> 2720
thermal             21792   -> 21644
udpbroadcast        6472    -> 6400
udpecho             7008    -> 6972
udprpc              8208    -> 8172
update_server       9292    -> 9252
user_leds           1420    -> 1420
validate            7468    -> 7468
vpd                 5608    -> 5608

(along with #2441, this is a building block for doing wider builds across multiple target directories; we'll need remapping of build directories to generate the same binaries in the end)