[MC] Fix assembler infinite loop on EH table using LEB padding.

Fix the infinite loop reported in PR35809. It can occur with GCC-style
EH table assembly, where the compiler relies on the assembler to
calculate the offsets in the EH table.

Also see https://sourceware.org/bugzilla/show_bug.cgi?id=4029 for the
equivalent issue in the GNU assembler.

Patch by Ryan Prichard!

llvm-svn: 323934