Fix up i386_sel4 arch after merge

Port muslc to seL4 build system and environment. Defines custom 'seL4' architectures for ARM and i386

remove the __mxcsr member from fenv_t on i386 to follow the glibc abi

in the previous commit sse fenv support was added, but there is no
need to save mxcsr (sse fenv register) so fix the abi incompatibility
with glibc.

preparing build system to handle ports - step 1