Fixes profiling for non-underscore-prepending toolchains
(elf, e.g., shared libs), and breaks a.out/ecoff toolchains.
May break mips kernel profiling too. Needs more thought, since the
original intent of __mcount vs ___mcount on mips date back to pre-1.0 days.
consistent with the (default) prepending of underscores to identifiers.
Because this reference is inside an ASM string it's too hairy to
conditionalize to support different toolchains that don't prepend underscores.
(Just don't do profiling with such toolchains.)
