R600: Use MCAsmInfoELF as AMDGPUMCAsmInfo base class

The main reason for this is that the MCAsmInfo class,
which we were previously using as the base class, sets
PrivateGlobalPrefix to "L", which causes all global
functions that start with L to be treated as local symbols.

MCAsmInfoELF sets PrivateGlobalPrefix to ".L", which is what
we want, and it is probably a good idea to use this as the
base class anyway, since we are emitting ELF binaries.

llvm-svn: 219237