Include the tool name for toolchain variables when using CROSS_TOOLCHAIN.

[jhb]

Summary

Previously variables such as AS, LD, etc. were only set to the path
but not the tool itself due to a typo.

Test Plan

• cross-building base/gcc after updating it to work with freebsd-gcc6 fails since base/gcc now uses these variables to set the corresponding <foo>_FOR_TARGET variables

[jhb]

Ping. This is only relevant when CROSS_TOOLCHAIN is set, so should be low risk. It is a dependency of D22594.

[jhb]

Ping @bapt