Do a clean build of lean-mathlib

[Yasaswinig7]

find . -name '*.olean' -delete;;
The directory (extern/mathlib) does not have any .olean files, hence delete is ineffective.

Before Fix:

$../../bench.sh sys lean lean-mathlib
------------------------------------------------------------------
 test    alloc   time  rss    user  sys  page-faults page-reclaims
leanN       sys   13.05 245660 30.57 0.63 0 60346
mathlib     sys   **01.38** 104656 1.33 0.18 0 44799

After Fix:

$../../bench.sh sys lean lean-mathlib
------------------------------------------------------------------
# test    alloc   time  rss    user  sys  page-faults page-reclaims
leanN       sys   12.98 256568 30.51 0.70 0 63592
mathlib     sys   **13.81** 351104 30.64 1.01 0 154038

[Yasaswinig7]

Updated the patch as per the feedback.