[profile] Standardise return type of profile_timestamp()

All consumers of profile_timestamp() currently treat the value as an
unsigned long.  Only the elapsed number of ticks is ever relevant: the
absolute value of the timestamp is not used.  Profiling is used to
measure short durations that are generally fewer than a million CPU
cycles, for which an unsigned long is easily large enough.

Standardise the return type of profile_timestamp() as unsigned long
across all CPU architectures.  This allows 32-bit architectures such
as i386 and riscv32 to omit all logic associated with retrieving the
upper 32 bits of the 64-bit hardware counter, which simplifies the
code and allows riscv32 and riscv64 to share the same implementation.

Signed-off-by: Michael Brown <mcb30@ipxe.org>

src/include/ipxe/profile.h

@@ -60,6 +60,8 @@ struct profiler {
 #define __profiler
 #endif
 
+unsigned long profile_timestamp ( void );
+
 extern unsigned long profile_excluded;
 
 extern void profile_update ( struct profiler *profiler, unsigned long sample );