Avoid use of the gnu specific "asm" keyword, and use the __asm__
keyword, as used everywhere else in the library. This fixes compilation
in C11, and unifies all uses of asm literals in the codebase.
Reported-by: @dragonmux
Fixes: https://github.com/libopencm3/libopencm3/pull/1425
Signed-off-by: Karl Palsson <karlp@tweak.net.au>