csu: Always use __executable_start in gmon-start.c

Current binutils defines __executable_start as the lowest text
address, so using the entry point address as a fallback is no
longer necessary.  As a result, overriding <entry.h> is only
necessary if the entry point is not called _start.

The previous approach to define __ASSEMBLY__ to suppress the
declaration breaks if headers included by <entry.h> are not
compatible with __ASSEMBLY__.  This happens with rseq integration
because it is necessary to include kernel headers in more places.

Reviewed-by: H.J. Lu <hjl.tools@gmail.com>