toolchain: do not use posix C code for asm language in gcc.h

Toolchain headers can be include in assembler files causing compilation
error for posix architecture. The fix involves adding
`!defined(_ASMLANGUAGE)` condition to exclude including posix C code in
this case.

Signed-off-by: Oleksandr Grytsov <oleksandr_grytsov@epam.com>