Prefer -Ttext over linker script values.

I found this while trying to build u-boot. It uses -Ttext in
combination with linker scripts.

My first reaction was to change the linker scripts to have the correct
value, but I found that it is actually quite convenient to have -Ttext
take precedence.

By having just

.text : { *(.text) }

In the script, they can define the text address in a single Makefile
and pass it to ld with -Ttext and for the C code with
-DFoo=value. Doing the same with linker scripts would require them to
be generated during the build.

llvm-svn: 305766