[ELF, LinkerScript] Memory region name parsing fix

This patch fixes a small issue with respect to how memory region names
are parsed on output section descriptions.  For example, consider:

  .text : { *(.text) } > rom

That can also be written like:

  .text : { *(.text) } >rom

The latter form is accepted by GNU LD and is fairly common.

Differential Revision: https://reviews.llvm.org/D35920

llvm-svn: 309191