[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/arch/arm32/include/bits/profile.h

@@ -16,7 +16,7 @@ FILE_LICENCE ( GPL2_OR_LATER_OR_UBDL );
  *
  * @ret timestamp	Timestamp
  */
-static inline __attribute__ (( always_inline )) uint64_t
+static inline __attribute__ (( always_inline )) unsigned long
 profile_timestamp ( void ) {
 	uint32_t cycles;