[boost] [context] Don't hard-code the assembler

AMDG This commit: https://github.com/boostorg/context/commit/7534fe5d1c01e7f543d86607770e9c46a... makes it impossible for me to run b2 --with-context msvc-12.0 outside of the visual studio command prompt. Please just use the assembler that the toolset provides. In Christ, Steven Watanabe