Mark ./configure as executable.
Mark build/platform.sh as executable.
Mark build/architecture.sh as executable.
Removed build/toolchains/gcc.d/.gitignore.
Removed unused wrappers from build/toolchains/gcc.d.
Removed old .gitignores.
Removed unused wrappers from build/toolchains/clang.d.
Removed `build/` rules from top-level .gitignore.
Add build/toolchains/gcc.d/available.
Linux builds :tada: -- Add Makefile.linux.
Update REAMDE.md to reflect new status of Linux builds.