add support for hosts that have custom symbol prefixes

add support for hosts that have custom symbol prefixes

From: Mike Frysinger <vapier@gentoo.org>

some architectures, like Blackfin, have a prefix added to all symbols (in our
case it's historical baggage)

while normally packages shouldnt care, when code starts mixing assembler with
C (like symbol versioning), you need to dip down into the inner details.
find attached a patch which queries gcc for the prefix (all gcc toolchains
have a builtin define of __USER_LABEL_PREFIX__) and then automatically has
asm() constructs utilize this

since most targets define __USER_LABEL_PREFIX__ to nothing, hopefully this
shouldnt cause any problems :)