Fix cmake syntax in generate_version_txt.cmake. (#96)

Commit 23f1d75 included `$(LLVM_TOOLCHAIN_C_LIBRARY)`, which
isn't valid cmake syntax – to interpolate a cmake variable you have to
use braces, not parentheses.

This led to a mysterious error from git

fatal: cannot change to 'rev-parse': No such file or directory

because once `${base_library}` is set to something that doesn't make
sense, `git -C ${${base_library}_SOURCE_DIR} rev-parse HEAD` substitutes
nothing at all for the source directory, and the command collapses to
just `git -C rev-parse` which indeed interprets `rev-parse` as the
directory to change into.

Author: statham-arm

arm-software/embedded/cmake/generate_version_txt.cmake

@@ -21,7 +21,7 @@ if(NOT (LLVM_TOOLCHAIN_C_LIBRARY STREQUAL llvmlibc)) # libc in a separate repo?
     if(LLVM_TOOLCHAIN_C_LIBRARY MATCHES "^newlib")
         set(base_library newlib)
     else()
-        set(base_library $(LLVM_TOOLCHAIN_C_LIBRARY))
+        set(base_library ${LLVM_TOOLCHAIN_C_LIBRARY})
     endif()
 
     execute_process(