Add abi flag to make-standalone-toolchain.sh.

Allow the invoker to specify a subset of ABIs for a given architecture when
creating a standalone toolchain.

Change-Id: I7b5dbce4d4c3eb9e3e38f4c0f54cdc7fa085f64d

Conflicts:
	build/tools/make-standalone-toolchain.sh
Replace -nostdlib with -nodefaultlibs when crtbegin/end are needed.

Using -nodefaultlibs allows the compiler to still specify the startup files
removing the need for manually wrapping the link line with crtbegin and crtend.

Change-Id: Ibb362325ff2aa663dc49ac7b715988c0dbbeb410