[yaml2obj]Allow number for ELF symbol type

yaml2obj previously only recognised standard STT_* names, and didn't
allow arbitrary numbers. This change allows the user to specify a number
for the type instead. It also adds a test to verify the existing
behaviour for obj2yaml for unkown symbol types.

Reviewed by: grimar

Differential Revision: https://reviews.llvm.org/D57822

llvm-svn: 353315