ld: testsuite: Associate C identifiers with symbols

Some targets prepend an underscore to C identifier, while others do
not (e.g. H8 vs. ARM). This can cause problems in two situations:
   1. when linking C and assembly code;
   2. when a symbol name is specified on the linker's command line.

This patch resolves both issues by using the asm keyword to associate C
identifiers with the corresponding assembler/linker symbol names in a
target-independent manner.

Signed-off-by: Jan Dubiec <jdx@o2.pl>