[COFF] Adjust secrel limit check

According to Microsoft's PE/COFF documentation, a SECREL relocation is
"The 32-bit offset of the target from the beginning of its section". By
my reading, the "from the beginning of its section" implies that the
offset is unsigned.

Change from an assertion to an error, since it's possible to trigger
this condition normally for input files with very large sections, and we
should fail gracefully for those instead of asserting.

Differential Revision: https://reviews.llvm.org/D38020

llvm-svn: 313703