Fix problem with PLT and GOT relocs on armel.
TinyCC fails to link correctly to libraries when both R_ARM_PLT32 and
R_ARM_GOT32 relocation to a same symbol exist (see
http://lists.nongnu.org/archive/html/tinycc-devel/2010-05/msg00032.html
for more details).
The patch marks all undefined weak symbols found in external libraries
as strong. The value of all remaining weak symbols is set to zero just
before the section is output.
Note by Thomas Preud'homme: it's been 2 months in Debian without any new
bug report, hence commiting.